Hi,
We use the MS agreement using full time equivalent numbers. This works out at around £30 per person but means we can license as many computers as we want. The agreement also entitles us to upgrades of the os we use. This is renewed annually over the 3/year fixed price.
We also add server licenses to the agreement and renew them every year. The god thing is we don't have to update the license count every time we get a new member of staff only when we renew. So sometimes it goes up and other times it goes down. Microsoft datacenter is also something I would recommend for licensing if you are virtualizing.