It takes practice judging yardages back from a target but it isn' t that much different than judging to a target.
The way you are judging, you find twenty and then probably find twenty five and thirty and so on....
Judging back from the target is similar to the last bit of judging towards the target. I start at the target and determine 5,10,15,20,...... back to where I am.
I almost always check my yardage guessimate by judging both ways. And sometimes guess the halfway distance.

But the most sure fire method is lots, lots, lots, lots of practice with a range finder and lots different terrain.

I guess the best way is whatever works best for you and you are comfortable with doing.

Also I know some others that bring a 20 yard rope and drag it around with them before a shoot to help locate twenty yards among the new terrain for that shoot.