When learning to ride slower, trying to hypermile, at that time I had no wattmeter. I cut a notch on the throttle body, and another at about 75% of the rotation of the throttle. Who knows what % it actually was electronically, but it allowed me to know if I'd let my throttle position creep so I could return to 3/4 throttle consistently. That mark corresponded to about 18 mph on the flat at the time. Holding on that same mark allowed me to climb hills with less wattage, pedaling hard of course to keep rpm above 15 mph.
If you have a wattmeter, then it gets much much easier. You simply aim to generally not exceed a wattage you decided on, and keep adjusting the throttle accordingly to keep below 300w or whatever you chose.
But really it just comes down to your speed. If you want to really stretch your range, 15-18 mph is the speed. 20 stretches it quite a bit, but it's amazingly more range slowing to 18mph. Below 15mph even better, but that's just too slow in most cases, for most ebikers. So it only really requires a speedo to stretch your range. Aim for 18 mph on the flat, and 15 up the hills. It might take more throttle to maintain 15 up the hill, but you will get it back going down the other side using no throttle for a bit.