The easy one: A few mornings in a row, right after waking up, or after taking care of whatever "has" to be done then going back to bed and resting for a while, measure your resting HR. That should be a few BPM below the lowest you'll ever see during the day, even if you've just been sitting for a while. The simplest way to get your HR while in bed is to use a pulse oximeter. They're available on Amazon for around $20 and the decent ones are quite accurate.
The hard one: Figure out your max HR while rowing. (For this use a chest-strap measuring device.) There are lots of different ways to do this, with varying degrees of pain, but the bottom line is that you need a good warm-up, go hard for a long enough period that you really get your HR going, go really, really hard for a bit longer, to just about max out, then if you really want the max, go insanely hard for, say, 500m more. The closer you want to get to your "real" max the harder it is. There are a few protocols out there, including on this forum of good ways to do it.
Then, based on your max HR, calculate your HR zones. (UT1, UT2, etc are defined as % ranges of that. See link below for more details.
https://www.freespiritsrowing.com/forum ... calculator
The PM won't tell you what zone you're in, but will tell you your real-time HR, if you have a compatible chest-strap sensor. (see C2 web site for more info on that).