There are no body measurements you can take to help decide what stem length to buy.
Best option is a professional fit on a bike, which is then easy to replicate on others. If this isn't an option, then you'll need to take several measurements off an existing bike that 'feels' right to you.
In my experience, the best way to ensure near identical fit on different bikes with different tube lengths and angles, is to use the bb as the reference point. This is the one point that everything else depends on, as it can't move, while the other contact points can. You need to ensure that your leg angles will be equal on any bike you ride. Once that is set correctly, then you can change stem lengths/height, handlebar reach/drop etc. to ensure you can get your back angle as close as possible to that on the 'feel-good' bike.
I regularly ride 5 different bikes, and all feel 99% identical despite ranging in sizes and geometries.