The top edge of the fin should come to just below the break in your ankle... IE: It should not restrict movement by jamming into the base of your leg, but should not be stuck on the end of the metatarsal arch.
If the fin is too large and floppy, then a smaller size (that will not come up as far) might be better. Crowding of the toes against the end of the fin (fin too big but with a short pocket, like Jet/Turtles) can cause cramping (this is the reason for many open-toe designs).
The position of the heel will be dictated by the design of the fin's sole plate.... a Jet/Turtle will have the heel fully exposed, while a Twinjet will have it mostly protected on the bottom.