Hi Daniel, as greg has pointed, there is a defined cover Strips size to be Screen Accurate . But in some cases you can exceed a "little" this measures in order to fit better but being still Screen Accurate.
arms and forearms - 15mm
Thighs and front of shins - 20mm
back of shins - 25mm
II would recommend when building an armor to look at other's Building Threads , High levels aprovals post, reference Photos and videos.
Cheers