So we are trying to find the length from the handle bar to the riders chest. The longer the length the more upright the rider will be.
Therefore I would get a broom handle or similar, mark off 630mm, then any other lengths you are considering say 710mm.
I would then hold the broom handle straight in front of me with my back against the wall and get a friend to measure the distance from the broom handle to my chest, I would then add the length of the stem to get my final measurement.
So for me with 630mm bars, the distance from my chest to the bar is 600mm.
with 710mm bars, the distance from my chest to the bar is 510mm.
If therefore I ran my 630mm bars with a 0mm stem I would require a 90mm stem to achieve a similar effect with the 710mm bars.
This is all a bit abstract and needs refinement but hey 😛