It’s fairly easy. I think there’s maybe two programs total that require 80% TM (going by memory, 5x5/3/1 and one of Kripteya’s phases), but in case you can:
-just to the normal TM test week, shoot for 7-8 solid reps. A 7 or 8 RM sounds about right for 80%;
-when you do the Anchor phase, on your heaviest week, note down your PR set on 1+ sets. Use the 1RM calculator, formula is: (weight lifted x reps x 0.0333) + weight lifted. This give you an estimated 1RM that usually is fairly accurate. Let’s say you do 5 reps with your deadlift at 150kgs on your 1+ week. Estimated 1RM is 175kg. You take 80% of that, which is 140kg. That’s your 80% TM - you want to make VERY sure, when you head into the TM test week after the anchor, work up to 140kg and see if you can hit 7-8 solid reps.
If you want to make sure, use the 1RM calculator and check a few numbers, you’ll see that 7-8 reps falls into 80% of your 1RM, give or take.
150kg x7 reps; 1RM = 185kg, 80% would be 148kg
150kg x8 reps; 1RM = 189kg, 80% would be 152kg
Do the math with different numbers and you’ll see that 7-8 reps is as close to 80% as possible. 7RM is a bit more than 80% (81%) and 8RM is a bit less (79%), if you’re really really OCD about it just take 1-2kg off from your 7RM and add 1kg to your 8RM but it’s really splitting hair at this point
EDIT: forgot to add a few points
-you COULD do, for example, a 5RM (85%TM) and take off 5% to get an 80%TM, as you mentioned. I personally just prefer to test thest the actual number of required reps because I’ve noticed that some people tend to be stronger in lower rep ranges or vice versa - i.e., in your previous example, you could to 115kg for 5 reps, then take off 5% which is 110kg. But you might not get 7-8 strong reps at 110kg if you TM tested it - that’s why I’d rather do the actual 7-8RM test.
-yes after the first cycle you can forget about your 1RM for quite some time. I’ve started with a 1RM (not TM, I mean an actual max) of 140kg in the deadlifts, picked a 90%TM of 126kg and added +4kg per cycle. Six months later I was doing 140x10, followed by a 2x3 at 152kg of Jokers. I knew I had it in the high 170kg range, but I’ve never handled weights close to that amount. The base of long term strength gains come from submaximal work, mostly. That’s why, usually, if your TM is set lower, you’ll STILL make progress. Obviously, not 50kg lower than it should, but if you need an 80% TM and it’s closer to 75%, it will still work.