I always use this site for calculating:
http://www.tyresave.co.uk/tyresize.html
Remember that "-5%" refers to a reduction in diameter, which corresponds to the speedo overreading by the same percentage (i.e. it will show 80 on the dial at 70mph), but you'll also be clocking the same percentage more miles than you're actually doing which is much less attractive.
I got this disastrously wrong when I went for a thinner profile on my V40 tyres last time, and ended up going from 5% over to 10% over... I've only done 22,000 miles but clocked 25k since then! On the plus side I actually get less mpg than my calculations because I'm recording more miles than I actually do. So I pretend it's a good thing

(also there's the minor plus point that shallower tyres have more rolling resistance and so they arguably corner better)
Speedo on the T4 currently overreads by 10% (with 17" alloys, not sure about the tyre but book says 245/45/17) so as I'm in need of 2 new tyres very soon, I'm going to liaise with my local tyre supplier and see what their calculations come out with.