How do I fill out 4f of the ATF Form 1?

I'm intending to put together a 12.5" SBR: I have the lower and the ATF Form 1 mostly filled out (including CLEO signature). I'm trying to figure out how I can measure the overall length of the firearm without the upper. I know its 12.5" at least, and can measure the stock (fully extended or... show more I'm intending to put together a 12.5" SBR: I have the lower and the ATF Form 1 mostly filled out (including CLEO signature). I'm trying to figure out how I can measure the overall length of the firearm without the upper. I know its 12.5" at least, and can measure the stock (fully extended or not?), but I cannot account for the length of the receiver. I've look everywhere online but it seems there are a number of different lengths possible, and I'd rather not guess and get this wrong.

Any help would be outstanding.
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