Shanamo, clearance between FH and tubesheet may be governed by crossover area as discussed previously.
Clearance between FH and shell head is governed by practical matters, such as adequate clearance to accommodate differential expansion between bundle and shell, fabrication tolerances, presence of lift or pulling lugs and so forth. Usually you just set it, rather than calculate it.
I doubt I'd make it less than a couple of inches.
Regards,
Mike
The problem with sloppy work is that the supply FAR EXCEEDS the demand