I never knew there were rules about wearing them in races! Learn something every day on here.
I suppose Bidders, it depends what you want to use it for. If you've just got it, you'll probably find it useful for data collection purposes if nothing else; over time this may help balance out the fact that lots of people find their race equivalent HR for a given pace to be higher than during training.
Dave, shf legs is right. Once you know what your true max is, you can manipulate the % in your HRM by entering a fictitious birth date. (I haven't bothered, just use the absolute numbers during the run, afterwards I can work out the % on a spreadsheet.)