Mattio,

You should do it for each fav. ie 1/$2 + 1/$3 = 0.83333

But from your figures I assume is over 70 races so we can assume odds of 264.6/70 = $3.78 per fav. This means there were 1/3.78 * 70 = 18.52 winners.

If these are tab prices I would be very careful about laying them as betfair prices are not always X% higher. Sometimes they are lower, mostly higher, and sometimes the favs are different.

If they are tab prices then its likely the number of winners should be 1.15% higher which would be 21.3 winners from the 70 races.

Again its based on the assumption this is 70 races for the figures you gave me.

Also my maths may be wrong so always double check it.