X2 That's how I always did it as well.

Gave me something to do during the sheer boredom of bow hunting when there wasn't a deer in sight too.

my cross bow has three pins. 20 30 and 40 yards. The diff of 10 yards from 30 to 40 can be 6-8" of drop. So 35-40 can be 3-4" of drop. Say your guessing range is within five yards, your impact is varying by as much as 4".

If you are just aiming at the side of a deer it probably doesn't matter (not recommended at all) but if you are aiming at say a heart, that is enough for a clean miss altogether.

Do you need a range finder? Depends what you are trying to hit. Some people are more particular than others.

I have the same trouble with my 45-70, its a real arc of trajectory too... so any distance you are off really changes point of impact.

I don't know about the rest of you, but I have spent enough time in the woods where I'm pretty confident in knowing 30 yards from 40 yards. And 10 yards from 20 yards.
I do use a range finder when I'm hunting big game with a rifle.

I have my crossbow sighted in at 30 yards. 20 is two inches high and 40 is two inches low. I will shoot knowing this and knowing distances from my stand.

That works for me most of the time. Sometimes a deer shows up somewhere unexpected so I range it. I'm not shooting at moving targets so there is time. I think compound shooters have some definite advantages over crossbow shooters, but crossbows have their own advantages. I got a crosbow to be able to take advantage of the generous doe archery season where I live, whereas our ML and rifle doe seasons were quite stingy. There was almost no learning curve going from rifle to crossbow and it has put a number of does in the freezer. I don't particularly like hunting with it compared with rifle or ML, but I wasn't going to take up a compound at my age.