I believe the site has made a mistake. If you are buying roofing or
roofing paint, by all means use the roof pitch factor to work out the
size of roof, but not to calculate the amount of water falling on it.

I am not qualified to argue but FWIW Approved Document H - Drainage and
Waste Disposal
(http://www.planningportal.gov.uk/eng...000000382.html)
also specifies, on page 40 para 1.3, a calculation which takes account
of the pitch.

For the OP there is also on page 40 table 2 showing 65m2 as the max
effective area for a 75mm outlet (subject to some other points). But
I've no idea of the practicalities/risks etc: I was just skimming the
document for something else. And I may have got it all wrong.

--
Robin