The idea is to have the 4 plumb bobs in line, one at each tip and each root, and also to measure from a rivet of your choice on the wingtip to a reference point of your choice on the tail.
When the measurements are the same, left and right, and the 4 plumb bobs are in line, your leading edge is as square to th fuselage as its going to get.
If you do not measure the distance from the tail, you may have a perfectly inline leading edge that is not square to the fuselage. In my case, I used a wingtip rivet, and the join line for the HS, that gave a measurement of around 5,115 mm. So, if I had one side that was 5110 and the other 5120, add them up and divide by two to find the "ideal" measurement and adjust the sweep to get it.
There is no hard and fast rule as to what your reference points must be, just make sure you use the same point on each wingtip.