I usually go 1 1/4" or 1 1/2" from the rail, depending on the toe and cant, and size of the fin…
8" up should do the trick, mine usually sit a little bit further back than that (around 7.5), at 8", even with the crack, you should be dandy.
glassons or boxed?
I glassed on my first fins ever, both crooked of course, the thing was magic…i ended up breaking one off at a beachie, put it back on straight; the board never worked the same again…
if youre looking for more down the line speed, go straighter and less toe in. the more you angle them out and the more you toe them in, the looser its going to be and the easier to initiate a turn it will be.