Like most folks, I have my pins set on even 10-yard increments, from 20 to 60 in my case. What I do for shots that are in those 'between' yardages, I visualize 10 increments between the bracketing pins. Say the target is 33 yards. I visualize my 10 increments and count up 3 with the 30 pin to get my holdover. If it's 37 yards, I count down 3 with my 40 pin and hold under. If it's 35 yards, you just split the difference and bracket the target.
How well and how quickly you can do it depends entirely on how much you practice, but one can be very precise with it. Shooting in the fixed pins classes, I have made Top Gun flight several times at local shoots, up amongst the open class shooters with their adjustable sights. It's kept me from winning a fewtournaments where they flight by score rather than by equipment, but who wants to be the best of the worst when they can be up with the best of the best?
The only shots you really need to learn the aim pointfor are those that are less than 15 yards.