The short answer - you're worrying too much. Relax - have a homebrew!
The traditional step-mash had 4 "rests" - temperatures where different enzymes did their best work. To understand our simplified "single-infusion" mash, we need to review what the last two stages of the step mash do.
John Palmer in
How To Brew Chapter 14 describes it great detail - much better than I could ever do.
The rest at ~140 to ~150 allows β-Amylase (Palmer's hedge trimmer) to work on the ends of the starch molecules. The rest at ~158 to ~162 allows α-Amylase (Palmer's pair of clippers) to chop up the big starch pieces.
These are the temperatures where each amylase works BEST - but they also do their work at temperatures outside this range. So during the β-Amylase rest, the α-Amylase is working too, but not at full efficiency. The opposite is true too.
Consider that as you slowly increase the temperature towards mash-out, you're going through the range (158-162) where α-Amylase works best!
Our single-infusion mash temperature is a compromise where both α- and β-Amylase both work pretty well - the guy with the hedge trimmer and the guy with the clippers are making equal progress.
None of these wonderful chemical reactions happens instantaneously - that's why the mash takes 60-90 minutes to complete. You need to look at where your mash spends the majority of its time.
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With a RIMS or HERMS system, you dough-in at 152. The temperature immediately drops to about 142, and after 15 or 20 minutes, the sweet wort exiting your mash tun has finally reached 152. Isn't that just about the same as a step-mash β-Amylase rest?
You mash at 152 for 40 minutes (or perhaps 70) - both β and α are working simultaneously. They're starting at different ends of the starch tree and working their way towards the middle.
You turn the temp up to mashout (168 or 170), and it takes 15 or 20 minutes to reach that temperature. Isn't that just about the same as a step-mash α-Amylase rest?
Think about it - aren't we single-infusion folks are really doing a step mash? OK, it's a kinda "sloppy" step mash, but it is one just the same!
Many folks agree that a light body beer needs 148 degrees, while a medium is 152, and a full-body needs 156 --- for the 40-to-70 minute period where both β and α are working together. Don't worry about the ramp-ups at either end.
So, the long answer - you're worrying too much. Relax - have a homebrew!
Dave