Having a rangefinder on your scanner is OK. It allows you to range landmarks such as a hill or a standing coyote. However, I move to my gun as soon as possible once I see a coyote. Yes, a coyote's eyesight is diminished in darkness, but don't be fooled into believing they can't see movement because they still can. If they couldn't, they couldn't catch their food at night. If you are using the rangefinder to tell distances, then you are probably waiting to move to your gun until you know the coyote is in comfortable shooting range. I am not saying this will cause you issues a lot, but it is a consideration that this late movement may spook coyotes. This is why a lot of people prefer to have the LRF on the gun for thermal hunting. However having it on a scanner is better than not at all.
For judging distance at night. Everyone gets excited when coyotes are coming or we wouldn't hunt coyotes, but let them come. I am probably guilty of not doing this as you see I typically stop coyotes at around the 125 yard mark, Pace off every shot after you start thermal hunting and you will start to get a really good idea of how big a coyote looks in your scope at different distances. If in doubt let the coyote or coyotes continue the approach. The one time to stop a coyote is when they are coming in "hot". It takes them awhile to stop, so stop calling and start yelling or whistling, or bark, etc because often they will run right past. Another exception to letting them come is if they are going to catch your wind. Then stop them before they get there. When a coyote starts taking up a significant portion of your scope, you will know it is time to stop them.
As for judging distance besides how big they look in the scope, what Rookiejx2 is referring to is something called color shading. This means you can see subtle heat variations in different parts of the body of the coyote. A coyote loses heat through it's head and lower legs the most. When a coyote is all one color, on most thermals, it is past the shooting distance where you can hold right on your spot. Again, different thermals are different, this is a general statement. I can tell you the animal detail is so sensitive on the Super Hogster that I had to change my yardages a bit coming from a Pulsar XQ38 and then a Flir PTS536.
Here are 4 examples:
#1
#2
#3
#4
Again, I feel the SH's 12 µm sensor hurts my examples here a little bit. In these 4 images, #1 you see virtually no shading. This coyote is around 400 yards. #2 you can start to see a little shading but it isn't prominent. The yardage on this coyote is around 325 yards. The yardage on #3 is at about 250 yards, and now shading is evident. #4 is about 150 yards. By the time, the coyote is at #4, there is no question it is in range.
How would I shoot these 4 coyotes? With my flat-shooting 22-250 zeroed at 1" high at 100. I would hold on the ears for coyote 1, right on the edge of the back for coyote #2, and hold right on where the brightest spot ends coming up the leg (this looks like the shoulder but it actually isn't) on coyote #3, and 4.