However, it is not calculating inspiration rank correctly. It seems like it calculates only to the next rank and stops there.

For example:
Khaz'gorite Pickaxe with R3 mats and R3 draconic missive has recipe difficulty of 440. My skill is at 335. And my inspiration grants 106 bonus skill.

So 335 + 106 = 441, which means I will be able to inspire to max quality of R5.

However, because I can only guarantee R3 without an insp proc, the addon says that the inspiration result will be R4.

Fixed in next release.