Need Notch Knowledge

We got the outer wall cut out today. Here is a pic of the shape we are going with.
I am roughly trying to follow the generic benroy plans found in the header links above, and now we need to cut the notch out of the hatch end of the wall. According to the plans, you are to cut a 3/8" notch from the wall starting just aft of the final roof spars. We aren't really following the plans using the same measurements as the generic, so how should I determine how deep the notch needs to be? I plan to skin the front, roof, and hatch with one layer of 1/8" birch, or one layer of 1/4" birch if we can't find 1/8" anywhere, if that info is helpful.
Thanks for the help on this fine holiday weekend!
I am roughly trying to follow the generic benroy plans found in the header links above, and now we need to cut the notch out of the hatch end of the wall. According to the plans, you are to cut a 3/8" notch from the wall starting just aft of the final roof spars. We aren't really following the plans using the same measurements as the generic, so how should I determine how deep the notch needs to be? I plan to skin the front, roof, and hatch with one layer of 1/8" birch, or one layer of 1/4" birch if we can't find 1/8" anywhere, if that info is helpful.
Thanks for the help on this fine holiday weekend!