The calculation part will be easy: (Systems with antivirus) * 100 / (# of all systems) will give you the answer in percents.
Needless to say that automating this calculation will help a lot. I guess a simple script can determine if certain antivirus software was installed and running. This could feed data into your security scorecard. If it is not the only indicator that you need to track, I’d recommend to look at the professional scorecarding software as well, like BSC Designer.
how to calculate the percentage of systems with antivirus/antispyware signature ?