Max dia hole in joist - measured twice, not sure about cut!

[Heeber]

(second attempt - wrong forum first time)

Hi folks,
next question from my bathroom project!

I'm ripping out a whole pile of boxing-in which hid waste pipes from the bath and en-suite (coming thru hole in wall). They ran along floor then down to main soil pipe thru a hole cut out of the flooring board - basically 40cm past end of where a 1700mm bath would end.

My intention is to drop them into the floor space under the bath, then replace the board and tile over.

Problem is that there's a floor joist between the 1700mm point and the soil pipe. I've read the Scottish bldg regs of max hole size of 0.25 times the Joist depth - mine is 170mm, so in theory a 42.5mm hole (for a 40mm waste pipe) would be the max. I just get the feeling on the forums that there is general reluctance to drill a hole this size - comments?

Alternative is going to be more boxing-in again :-(

thanks,
Heeb