This is really a 3 step process. First thing you need to look at is conversion efficiency, this should be at or really close to 100%( for decoction you can actually get above 100 as you are busting hard starches that normal infusion can't.).
Then you have mash efficiency, which when you het higher conversion efficiency, is just recovering sugar from the tun.
The you have brewhouse which is basically how much you lose in the brewhouse to the fermenter.
I am a no sparge brewer and my values are as follows. 100% conversion, 90% mash, 85% brewhouse.
I always try and keep my water to grain under 2.9qts/lb, employ a step mash (standard Kunze 20-64c,30-72c,10-76c@5.2pH), barke malts, the use of sauergut, and constant recirculation(herms).