You usually hit A milliseconds before the timeout; which would be on the last beep or before, depending on how fast you receive the sound.
Still, in most cases the delay you hit will be a bit of (I'm, for example, always 20 units too early). If your are using the EonTimer, you may tell it which one your hit (Delay Hit), so the timer will adjust itself according to your personal timing. You can do it manually, too: Just look up which seed you have hit and tell the timer you want to try to hit it later. For example: If you want to hit the seed with the delay of 617, but somehow always hit at 597, you may try 637 next time.
I've got a question for myself: The DPP guide is referring to Monster Frames. What are these? I always screw up the termini because the word "frame" has loads of meanings. Let's take the unit of the delay for example: I always though of them as "frames", for the refresh rate is 60 frames per second. Unfortunately frames also adresses the offspring of a seed, and now Monster Frame?
We suspect it is equal to Encounter Slot, isn't it?