The warmup should give you a feel for what percentage to adjust the intensity to. But your plan of around 80% sounds reasonable. As long as you get at least five complete steps of the ramp done, and don’t finish the entire ramp, you’ll get good results.
The ramp part of the test is designed for you to fail at some point, as it’s attempting to measure your maximal aerobic power. If you don’t fail to complete the ramp, then your MAP will be calculated too low.
The test will still work if you turn down the intensity percentage at the start.
The intensity percentage changes the power targets throughout the ramp. Your results are based on the power levels you reach. It doesn’t matter if you achieve those levels earlier or later in the ramp.
The only caveats are that you have to complete at least five ramp steps, and also that you fail somewhere before the end of the ramp. (See link to post below for clarification.)
The first part (five steps) is based on the fact that HM doesn’t report MAP numbers until the start of the sixth step. I admit that I am making the assumption based on this that it needs at least five minutes of ramp data to calculate a MAP.
The second part (fail the ramp) is based on the fact that the test expects you to fail. If you don’t, then you won’t have reached your maximum power levels, so the calculated MAP will be too low.
I wasn’t able to quickly find any official documentation about either of these points. So everyone should take what I’ve said with a suitably sized truckload of salt.
The good thing about HM (as opposed to FF) is that you keep climbing the ramp until you can’t go anymore. So, your numbers don’t matter too much, as long as your FTP is not too low. My first ever ramp test used an estimated FTP which was way below my actual FTP, and i had to climb up way too many steps.
80% sounds like a good estimate; I’ve dropped mine by a similar amount after a long stint off the bike.
Good luck! And well done for getting back into it after your recovery!
I used to think the MAP value from HM was average power from the last 5minutes of the ramp, but I just had a look at a bunch of my previous HM results and it looks like for me its using my 3m30s max power as the MAP score.