I know it’s been a long time since your post, but do you happen to remember the tension given for the Gates Krikit gauge. I have one of those and it I think it would be far easier to check the belt tension with one of those.

Is it just me or does the belt tension on an M8 seem to be tighter than that of an old twin cam like my 08 Electra glide.

That's right Neale, it is tighter on the M8s. ( Not sure if the Heritage set up is the same as the EGs)  Without having a look at my manuals I know my 03, 07 and 2012 Ultras definitely had a looser tension. I do have one of HD belt tensioner gauges somewhere, prolly in my bottom drawer, but I don't use it any more. I just do it up tighter than I used to. Maybe my 2020 Ultra has about  10mm ?  I can check it for you if you want. When ever I remove and replace the rear wheel I use a bit of duct tape or masking tape from the swing arm to the axle cam washer, mark a straight line between the 2 parts, cut the tape at the gap and line the marks up when I replace the wheel. Dunno if that makes sense to you, but I know exactly what I mean ! 

Just looked at my owners manual and for my 2020 Ultra the deflection should be 9.5 -14.3 mm.

Thanks for the reply. Appreciate it.

And cold not hot.  Do you have the presssure tool as well?

I heard that a rule of thumb (if you do not have the gear) is to twist the belt to 90 degrees.... thats pretty close to the right tension. 

I’m probably just overthinking it.

That's better than UNDER thinking it.