For the cavities and score cuts I just measured from the outside cuts 3/16" in toward center, like for where the belly pan and side walls index into the wing section on the bottom. There are 8 or so tabs that have to fit in, I measured from the outer tab cut line in that 3/16" or whatever your FB thickness is, to get the right tab cut. Remember your outer dimensions no matter how much you scale up are gonna be consistent, the difference is the thickness of the FB compared to the enlarged cut outs on the plans... if that makes sense.
I could make a video on it sometime but it is actually pretty simple to figure out. It was a easy little guideline for me to follow for my first time and it worked out for me