292 or 291 mm spokes will work fine. The first wheel build can be a bit worrysome since there seems to be a lot of variables and money to spend before things get started but there's more room for error than you're concerning yourself with.

More direct to your question - everything you're dealing with is simple trigonometry. You can figure out spoke lengths with a calculator and a pad of paper if one were so inclined but when you've got a problem like this one where you're not certain you've got the right answer try drawing it out in a right triangle and see how much something like 2 mm of spoke be offset will change the necessary length. The answer is really not much.