What length bolts to run with 15mm spacers?

[Shavey]

Want to run 15 mm slip on hubcentric spacers on the rears of my 1.6TD wagon. I've measured current bolts and the thread is 28mm long. Cant decide whether to get 40mm or 45mm extended bolts. With the 40s I loose a turn or two when the bolts are done up, and with the 45s I'm not really sure whether they might run too far into the drums? I'm guessing that there's more than 2 or 3 mm tolerance in the drum threads before the bolts could snag brake components but not really sure. Whats your advice guys?

Thanks