One of the interesting things that Bryan has turned me on to in the recent months is tracking the percentage of extract you get after each rest. We use an adapted form of Kai's first wort gravity chart in our spreadsheets and we have shown that for 100% conversion we nail that gravity consistently.
What does that mean? Well we know that we can expect 100% of the available first wort extract at the end of our mash. Since we are using no-sparge, that turns out to be our pre-boil gravity. In order to track extract across the mash, you simply take gravity readings with your refractometer after each rest. What you'll see is the progression of available extract for a certain temperature and time.
For instance, we both use sight glasses so we can SEE conversion happening and gravity readings confirm this. If you know that you expect 12.9 Brix at the end of the mash and you read 5.2 Brix at the end of the 144 F rest, you know that you have obtained 42.6% of the first wort extract from that rest. You could certainly infer that beta amylase was very active at that temperature. If you then show 8.5 Brix after your 147 F rest, you know that 23.3% additional extract has been gained, and so on and so forth. Bryan has been hitting ~90-95% of his pre-boil gravity after his 3rd beta rest.
You can also use this as a troubleshooting tool for poor conversion or even high gelatinization temps for new malts. If you run a multi beta amylase rest schedule and your first rest is 147 F with a step to 153 F, and you don't see any extract content until the ramp to 153 F, you know that your gelatinization temperature lies somewhere on that spectrum. If you see sluggish gains in extract content throughout the mash, you may have poor conversion.
For those using single infusion, this troubleshooting tool could also be used to determine whether you should extend rest times. For instance, if you plan on mashing for 45 minutes, but at the end of the duration you only have 65% of the deisred extract, you can extend the rest time and take another reading at 60 minutes, 90 minutes etc. and use that as a metric for that temperature rest.