Having a wild stab I presume you are referring to proportional ETA corrections based on quarter/half/third/etc markings on your track.
You make appropriate markings on your chart at equal intervals on the leg, i.e. a halfway mark or at thirds, quarters, etc. Ideally look for a good feature that is as close as possible to a proportional point. You then work out what time you would arrive over those points in still air.
When you are flying your route take note of the time you actually pass over the point, from that work out the difference between the still air figure and the actual time you flew over it. This is where the inverse proportion bit comes in.... you multiply the difference by the inverse of the distance of the point over the whole leg. So a halfway point is 1/2 of the leg, the inverse of a 1/2 is 2 therefore you would multiply the still air/actual difference by 2. If a quarter of the way in you would multiply it by 4 and so on. This gives you how early/late you should be at the end of the leg assuming a constant wind velocity.
A quick example; you are flying a leg on a nav route, in still air figures you expect to fly over your halfway point at 5 minutes. You pass over the halfway feature at 6 minutes, so you were 1 minute late. Working on the idea that the error will be proportional you will be 2 minutes late at the end of the leg (1 minute late halfway, inverse of a half x one minute == 2 mins).
I use this method along with the Max Drift rule for wind correction and it all works very well. It is always accurate assuming a constant wind velocity. Be careful about any errors you induce through starting your stopwatch late and such like, one off errors are not proportional
I also apologise for a rather butchered explanation, best I could do at this time in the morning!
ASI