John,
If you mean steel strings then
this is the calculator I use.
If the guitars with scale lengths you quote were in EADGBE tuning with 12-53 strings then the 660mm scale length one would have a total string tension of 169.68lb and the 650mm scale length one 164.58lb.
_________________
Dave White
De Faoite Stringed Instruments". . . the one thing a machine just can't do is give you character and personalities and sometimes that comes with flaws, but it always comes with humanity" Monty Don talking about hand weaving, "Mastercrafts", Weaving, BBC March 2010