The way I was taught:
Adjust the forestay/headstay to get the desired mast rake.
Begin tensioning both the headstay and the backstays, maintaining rake.
Stop when headstay is at the desired tension (15% is pretty typical, but some may go as high as mid-20s).
Split backstay tension will be whatever it is. The goal is to maintain the desired headstay tension and mast rake. Because the angles on the backstay are different from those on the headstay the tensions won't be the same, so you can't really say 15% all the way around. Also consider whether or not your spreaders are swept back. If they are your shrouds will also contribute to headstay tension.
If your backstay is split all the way to the mast head
you have redundancy in case of failure. You can also rig a couple of blocks and a tackle as an inexpensive backstay adjuster
so you can play with tension, mast bend, etc. while sailing.