ATM Primary to Secondary Distance Calculation

["Al Klayton" <aklayton@pcisys.net>]

Over the past few years I've attempted to upgrade my 8" f/6 Dob by replacing
the original low profile helical focuser with a JMI NGF Mini-3, and by
replacing the composite wood-like primary mirror cell with a well ventilated
all metal cell from University Optics. The secondary is 1.52".  I think I
tried to mount the new cell so the mirror was the same distance from the
secondary as "before", and the scope does seem to come to focus with "room
to spare" as far as focuser travel is concerned. However, I'm now wondering
if in fact the distance from the primary to the secondary is correct.
Given the optical system as described (8" f/6, 1.52" sec), how would I
calculate the correct primary - secondary distance? For visual use of the
scope, how critical is this distance?
Thanks,
Al