Conditional Formatting help.

[mikel301292]

Im creating a planner for my local GP which, ive got a small problem with. Basically on it its got, the ammount of appointments each Gp has, and above it has whether they are "in" on that day or "off". But the Problem is that when someone is on holiday, a "h" goes into the box, i want this then to turn the box with the ammount of appointments in to "0".
I've tried using the basic "IF" fuction: =IF(A1="H",0), which makes the box go to 0 when there is a h but i cant type any other number into that cell because it will over ride the forumlae in the cell.
I then tried to type the same forumlea into the conditional formatting section but that just does nothing.
If you have any way of solving this problem please help me. Thanks!