Man there are a lot of "what if's" in that question.
If I had to give a hard fast answer 35% of the body length to the rear of the axle.
My first 5 x 10 I used the 40% to the rear and had handling problems above 62 MPH. I later extended and reinforced the tongue and the handling problem ( side to side sway)went away. So was my axel placed wrong or was the problem in the towing geometry ? I weighed "Chubby" a month ago fully loaded ready to camp 1830 #

Tongue weight is 280 # that's a little over 15%. If you are towing with a small vehicle, building first might help you tune in on the tongue weight but with a larger tow vehicle it is not so critical. It would be a shame to get done and then place the axle only to find it interferes with the doors or you now have an ill placed window or other feature. I feel the frame dictates how the build should progress.