There are several ways to figure out the actual gravity change. The easiest (no real calculations) is to use the fast ferment (or pull a sample) and add maybe 5% TS. See where that puts it. Be advised, It doesn't change the actual gravity a lot, but it really does dry it out on the pallet.
I avoid using sugar unless it's really needed, and even then I limit it to 10% of the total gravity contributions (unless it is part of the recipe i.e. DFH 120 min). An ipa handles 10% TS well, but other styles are more sensitive to the hot alcohol. Of course, many other styles benefit from some body anyway. Since the effect is specific to each beer, it becomes more of a 'got a feel for it' type of thing.
Sometimes I'll even brew a 'sessionable' iipa by adding body. Not cloying, just full. It slows the consumption rate and I like the clash of malt and hops. It may be out of style, but I'm brewing it for me.
Please understand, if a beer finishes a touch high I will not add TS (ipa is the exception). It's not worth the way it thins. It seems to reduce the malt backbone. A little TS goes a long way. I liken it to trimming the hedges with an 80cc chain saw. Can it be done? Sure. Can it be done well? YES. Is it a lot easier to make the bushes look like crap with such a tool? You know it. Will it ever be as good as a pro with shears? Hell no!
I guess I'm saying all this because I'm worried I may mislead someone to thinking that TS is a magic wand that has gotten a bad rap for no good reason. TS can not replace good planning, good ingredients, and good process control. It is just something that I find handy, but only once I knew how to control everything else (in theory). Only then did I start to experiment with this.
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