Can you tell me how to apply a variable % penalty to corrected times

I am approaching you on behalf of PYRA (Poole Yacht Racing
Association)in the UK. We run 2 classes under IRC and a third under
the Poole Harbour Rating System. It is the third which is giving me a
problem as we apply a 1% penalty for each crew in excess of 2. ie 6
crew = 4% penalty,
ie (corrected time)*(100 + (number of crew - 2)/100).
This may be differenct for each race.
We have recently started to use SW and still very green and would
appreciate any suggestions as to how this might be done.
If it helps our web site is:-
http://www.parkstoneyc.co.uk/pagepyrahome.html
Mike