if you go with 72 times diameter your splice will not likely be the failure point. Also, with an 18-inch bury you can make the taper 12" by cutting off one strand every inch or 6" by cutting a strand every half-inch. I would not use less than a 6" taper.

Also, when measuring you need to account for the following issue: You will loose a few inches in length due to the expanded diameter of the bury. Test how much loss by measuring a mark on the rope to the end of your loop (before the bury stage). Then complete your bury and milk it out, and do a test pull and measure again. It will be shorter.