18.5" * 20 = 370" to get the size for the full size plane, since it's 1/20th the size of the original.

then...

370" / 3 = 123.33" for 1/3 scale

370" / 4 = 92.5" for 1/4 scale


A final formula to sum it up..

(scale length / Original scale) * (new scale)


for 1/4 scale:

(18.5 / .05) * .25 = 92.5

where .05 is 1/20 and .25 is 1/4

HTH

Ed