[Yetiman]

There isn't really one right stock answer, for optimnum results you need to experiment some.
Same goes even for spark plug heat range. As you start modifying the engine and using different ignition components there's no answer in a book.

BUT, with a healthy ignition system, as a base starting point, if you have less than 10.5:1 compression, try .050 to .060.
Between 10.5 and 13:1 try .040 to .050.

If your ignition isn't so powerful you will need to go with a slightly smaller gap.