Dana,
sometimes it helps to convert to inches first.
if the 1:1 plane has a 103 foot, 9 inch span (like a B-17, say)
the wingspan is (103*12 +9 = 1236 + 9 = ) 1245 inches
if you divide 1245 by 48 you get 25.93 inches,
which would be the wingspan of a 1/48 B-17
if you divide 1245 by 72 you get 17.29 inches for 1/72 scale.
1245 / 144 ~= 8.65 inches for 1/144 scale, etc
Scott,
i find it easier to work from the 'real' dimensions,
so to go from 1/76 to 1/35 i would multiply by 76 then divide by 35.
(the end result of which is identical to multiplying by 2.17, as Pix said)
but breaking it up into 2 steps may make it clearer.
once you 'understand' it, you can do it which ever way you like better.
ed.