First of all, it's not craft just any old item you are optimal at, it's craft an item you are just barely optimal at. If you have 203 spellcrafting, find a spell that becomes optimal at 200 spellcrafting. When you reach 210, look and see if there's a new spell that's optimal now.

Next, you have to look at the formula: (Construction exp + decon exp) / (resources used - resources returned on decon) to find the best exp per resource used (the larger the number the better). It may not always be the item you are just barely optimal at, there are always exceptions. Test a few different items around your current skill level, see which one gives the best exp/resource and run with it.

Oh, one more thing. At optimal you get back 1/2 the resources on decon. If an item has an odd number of resources it rounds down - so if you have a choice between something that takes 5 resources (decons to 2) and something that takes 4 (decons to 2), choose the item that takes 4.

Hope this helps,