Just set it and check. For me it was loss of 100W if we play numbers game.
I am doing exactly the same thing post a Heart Attack at the end of July and then subsequent surgery to repair my femoral artery after the angioplasty.
I started at the MAF calculatiion also, but have settled on 125 which is slightly above that and closer to my old endurance ride level. Power is down a lot, but It is encouraging to see the PWR/HR steadily improving.
The Trainerday app has the ability to control resistance based on a target HR which is really useful at the moment
I am trying that Trainerday feature, first two attempts It did not work properly but I am seeing on their forum and Alex is very helpful
Because my FTP and Threshold HR were invalidated overnight, I had to play with my old numbers to get the app working properly at my new low outputs. One workout sorted it out.
Now it works fine, and works best with just selecting the HR you want to target for the work part of the workout, and leave the app to handle the warm-up