What Leon said. And...
Having hung a few hundred slab doors into existing jambs or site built jamb
s, 1/4" is correct. First, if you are starting with a slab door blank of a
ny quality, it will have a bevel side and a hinge side. The hinge side wil
l be square and the bevel side (lock side) will have a 5 degree angle cut a
cross its thickness. The bevel allows the door to close even after some mo
vement as the "square edge" is gone.
Measure the width of the door from the square side to the wide side (which
it sounds like you have) and that side should be 1/4" smaller than the jamb
, the other side about 3/32" less than that.
When the butts are mortised perfectly flush, not high, not low, then they h
inges will space out 1/8" from the jamb, so 1/2 of your 1/4" allowance. Th
e remaining 1/8" will allow the same 1/8" reveal on the lock side, so your
1/4" gives you a perfect 1/8" reveal all the way up and down the sides.