From a search I saw the consensus it 3 inches shorter string for the bow length.  
So if I have a 59 inch long bow the string should be 56?  
When would 54 inches be appropriate for this bow?

Its a Morrison ILF mini 14 riser and trad tech black max longs which are 62 on 17 riser so I figured 59 on a 14.

Also I have a Reflex Traditions(poor mans Gamemaster) 21 inch riser with shorts.  If I use the longs I have on it would I get a 66 inch bow and a 63 inch long string?

Thanks, new to the switching limbs stuff.

You should be right on with a 3" difference.If you are only 2" shorter,you will have to twist the string too much.

The 3" shorter then the bow AMO length is a general rule.I like to actually measure from nock to nock to make sure for when I make a string for my bows.

I know for my Silvertip recurves the it is 5" shorter than bow length.

I believe if a bow is made to AMO specs, the rule is a string is 3" shorter for longbows and 4" shorter for recurves.  Again, JRY309 is correct...measure it and be sure.

Short version, AMO/ATA says 3" shorter, longbow or recurve, measured under tension. To be more precise, the bow is supposed to be marked 3" longer than the "string master" that puts it at the proper brace height.

It's going to boil down to how the bow was made, how it was marked, and what your preferances are.  3"-4" shorter for the string is a good starting point, but not a guarantee.