G'day Oz...
Yeah, it is only a guide - That is, 30/60ml in 25-30Sec (including pre-infusion time). In my view, as a guide this means plus or minus 5Sec, not all the way out to 40+Sec. The longer you leave the coffee puck in contact with the brew water (including pre-infusion) you are more likely to capture more of the bitter elements from the bean, hence the guideline of 25-30Sec (25Sec in fact). That time wasn't arrived at by accident, it was obtained after a significant amount of scientific experimentation many years ago, so I think it is a guide worth following but not necessarily with absolute rigidity.
Just my thoughts...

Mal.