In the Half Monty test, when we are told to keep heart rate in a specific range by adjusting the power, isn’t that also a function of cadence?
In my personal experience, to keep my heart rate in a specific range, I can lower my cadence and increase my power at the same time (by changing gear in level mode) and I do see about the same heart rate. That’s why when I perform the half monty test, my gear selection changes the results of my FTP. Does anyone else have similar experience?
I do find that I can maintain the same power for lower heart rate, or higher power for same heart rate, if I drop my cadence and up resistance but the difference is much more marked around and above “FTP”, not so much at the Half Monty tempo level (which is all aerobic/slow twitch). I believe the advise for the Half Monty constrained section is to ride at your typical/preferred cadence, just as you should for the FF 20 minute. For me that’s around 88rpm but I vary it up/down slightly to keep the HR level while sticking to a fixed resistance (no gears on my Wattbike).
Of course, now you’ve said it I’m tempted to try the thing at 60rpm and see what happens! 
Totally agree. It is a choice we can make to lower heart rate by using more low cadence ‘strength/torque’ to get the power rather than making our aerobic systems work as hard (there’s a cost to both of course).
Just do it at what you would normally ride at if you want it to represent what you will do on the trainer?
The number is only for you and your settings for future workouts, so maybe think about cadence in the context of normal workouts so that the setting the test gives you then aligns to what you’re going to train at.