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This is the way I do it. Multiply prototype distance in feet
to get prototype inches. Then divide prototype inches by 20.3
I do this on a excell spread sheet. Column A is Proto Feet, B is proto inches and C is the dimension is 1:20.3 scale.
Use a formula to do the math for you so you can go from 1 foot to whatever and fill in the column.

Example a building is 24 feet by 36 feet.
(24x12)/20.3= 14.187 inches or 14 3/16 inches
(36x12)/20.3= 21.281 inches or 21 1/4 inches

To build this building at 1:20.3 scale you would make it
14 3/16 inches by 21 1/4 inches
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