Well, there is no set way to hit a specific delay... You're only human. 1 Delay is 1/60 of a second. However, here are some tips.
Try to be consistent (i.e. Always button mash, so my range on Diamond is about 620-628, fractions of seconds, which are easier to hit than a delay of say, 700) with hitting a particular delay.
Actually, did you calibrate (do multiple test trials) and estimate your average delay range? Delay varies from versions, individuals, and whether or not the GBA slot has a game in it.
Also, have no distractions (however, some say that music or a timer works for them), ensure that your game character has not connected to Wifi/Lan as the last activity (just resave to remedy this), and have some patience/luck. :P
I follow Smogon's instructions with a few tweaks. They suggest that you subtract one whole minute and disregard seconds from your target time (e.g. Target: 03:15:37 becomes Starting: 03:14:00). Instead, I set my clocks (DS and computer) to 03:15:00, then wait about 15 seconds or whatever, and turn on my game, which takes another 15 seconds or so to boot up, to hit the 37 second mark. This is just an example, so judge your own time.
I assume you have no problem with frames, just delays?
Awesome. It's been a pain for me to time my delay. And it just seems like it doesn't work. Well, I got the egg natures part, but the IVing is a pain. Do you have a certain way of going through with your RNG breeding or do you go with what the instructions say?