In order to pitch the support arms for the flight-deck broadly between the gaps made by the swept area of the propellers, it's been produced as a template 12.5" square. Then there's a margin drawn around its perimeter to represent the 1" width of those arms.
This reveals that allowing for the inserts at either end ~ which together account for 3/8" ~ that we need arms of around 16 & 7/8" in length.
