If your scope is marked as "1 click =1/4 inch @ 100 yards", you have a minute of angle system turret (approximately). One MOA is actually 1.047 inches at 100 yards, but this little bit of rounding off won't hurt for the distance you are talking about. At 500 yards, one MOA is roughly 5 inches. You mentioned 68 inches of drop for that distance from your 100yard zero. Divide 68" by 5" to convert inch drop to MOA drop, then multiply by 4 (to find the number of clicks if you want to count clicks). Or if you turret has MOA markings, just dial the MOA (13.6) which you would round off to 13.5. Be sure to shoot this solution for real on the range, because that is the ONLY way to be sure it works. I don't know how much experience you have, so if you have not heard of "trajectory validation", you definitely want to check it out. Drop charts are just paper until PROVEN in the field. Have fun shooting!