The Casterlevel listed in the item descriptions is ignored. If it is needed assume it is equal to the minimum level required to cast all spells required by the recipe or is equivalent to the other requirements needed. If the item was created by these rules it’s level is the casterlevel of it’s creator at the time it was crafted.
The DC for the Spellcraft check is determined by the category of the item:
Minor Item: DC 12
Medium Item: DC 18
Major Item: DC 24
The Category can be determined by using the lists in the Magic Items chapter. For weapons and armors minor items are those with an enhancement bonus of 1-3, medium items those with bonuses of 4-6 and major items are those with bonuses of 7-10. The GM has the last word on what category an item falls into.
All requirements can be circumvented by increasing the DC by 5 for each ignored requirement. This includes the “caster’s level must be at least x*bonus” requirements.
The character may NOT take 10 on the spellcraft check. The penalty for missing the DC changes as follows:
The difference between the check and the DC is multiplied by 10%. The resulting percentage of materials and time invested are lost. By paying the missing cost and spending the missing time the item can be completed. A new check is made after the new crafting finishes with the original DC lowered by 1 for every 10% of the item already completed.
If the new check is closer to the new DC than the original roll to the original DC the item’s crafting is progressing. Calculate the completed percentage just as for the first roll (multiply the difference by 10%). If the roll is not closer to the DC than previous rolls no progress was made and the partial work was wasted. But unless a check is missed by 10 or more the half-finished item remains simply unfinished.
If the DC on any roll is missed by 10 or more there is a chance that a cursed item was created, or all invested material and time are simply lost (GM’s choice, roll for the player in case you want to incorporate cursed items).
All rules not mentioned here are left as is, for example adding +5 to the DC to cut time needed in half etc.
For every additional 10% of the item’s material cost supplied the DC is lowered by 1. The increased material cost is the new base for all calculations if the item is left half-finished. The time required for crafting does not change. These additional funds represent higher quality materials, outside help or reference works and the like. This rule cannot be used to pay less by increasing DC.
Aranian the Mighty Wizard wants to create a headband of vast intelligence +4. This is a medium magic item (as it is listed on the medium wondrous items list), so the base DC is 18. Unfortunately he is unable to cast fox’s cunning, so he ignores this requirement by adding +5 to the DC, resulting in a final DC of 23. The item costs 8000gp and takes 16 days to complete. He pays the gold and works for a little over two weeks and then makes his spellcraft check with a modifier of +12. He rolls a 8, resulting in a result of 20. He missed the DC by 3 and his item is left unfinished. It is 70% completed though, so he coughs up an additional 2400gp (30% of the original materials) and works for another 5 days (30% of the original time). His DC is lowered by 7 (the completed percentage divided by 10) and he can hardly fail his check, thus completing his shiny new headband.
Aranian the Mighty Wizard wants to be able to cast as many 9th level spells as possible once he can cast them. So he prepares to create a pearl of power to recall one of his 9th level spells he can cast with his next level.
The base DC is 24, but he wants it finished before reaching level 17, so he chooses to speed things up a little in addition to ignoring the requirement of being able to cast 9th level spells. This brings the DC to 34. He has some spare gold from a recent adventure and he decides this is a good time to use it. He decides to spend an additional 40% of the material cost: 40,500gp + (40,500gp * 0.4) = 56250gp total. This lowers his DC to 30 but he will still need only 41 days to craft this item.
His spellcraft skill is now at a +19, which could be better, but an adventurous wizard has to sometimes be able to traverse water and walls the old-fashioned way, too. So he rolls and gets a 5 on his roll, for a total of 24. So he’s only finished 40% (100% – [(DC 30 – 24) * 10%]). Time to roll up his sleeves and spend more time crafting. His new DC is 26 (30 – [40% / 10]) and he needs to spend an additional 33,750gp (60% of 56,250). Finishing the item will take 25 days (60% of 41 days). So he rolls again and rolls another 5, for a total of 24. Still not finished! But at least some progress: The difference between his new roll and the DC is only 2, meaning he is now 80% done. Spending yet another 11,250gp and 9 days to finish the remaining 20% allows him to make another check (against DC 22) and hopefully finish the item.
Reasoning and possible Changes
This rule simplifies the existing rules and gets rid of the dreaded casterlevel-may-or-may-not-be-an-absolute-or-optional-requirement problem. With these rules it is still fairly easy to create magic items without much risk. But by not allowing Take 10 and adjusting the penalty success is often not guaranteed while a missed roll is not a complete desaster.
You could easily make crafting more difficult by increasing the base DC, resulting in an item needing more steps to finish. Or you could increase the lost percentage to for example 20% (missing the DC by 5 completely wastes material and time) making a missed roll more painful thus putting mightier items out of reach of characters not willing to take great risks.