The issue is that there's no advantage in turning trains around in (say) 21 minutes vs 25 minutes as you'll not be able to use the platform for the extra few minutes.
As I understand it (unless you play random platform allocation) the simple way to manage usage is to have a pattern which allows you have 2tph, 3tph, 4tph, etc. using each platform.
That means 30 minute, 20 minute or 15 minute blocks to work with, with the 5 minutes
@Bald Rick mentions that means either 25 minute, 15 minute, or 10 minute turn arounds.
Whilst 25 is probably a bit long, 15 may be more of a challenge to ensure when things are running quite as smoothly as we'd like.