Here's my $.02. The advice given above is good. I'd add that it is wise to check the "zero" of each of the "4" cross hairs. Typically these are indeed graduated off in 10 yard increments, but only approximately. Most crossbows are so fast that the top cross hair is going to be extremely close to "dead-on" from near point blank range to about 20 yards. But it is wise to verify.

For example my Tornado and scope combination is set-up absolutely dead-on at 20 yards. And the Nikon BDC scope specs are supposed to be "pre-calibrated" for 20-30-40-50-60 yards. Truthfully it is a tad high at 10 and 30. About 2" high at 40. About 1" low at 50. And about 2" low at 60. Certainly close enough for a clean kill on deer sized game easily out to as far as I am likely to take a shot.