Balance force components?

[BigDuke66]

Well over at shrapnel someone suggested for a game some % numbers to keep the components of a force in reasonable boundaries.
To be honest I didn't pay much attention to it as I was doing only scenario games till now, but now I think well maybe that idea is not too bad, before one stumbles into a totally crazy composition like 80% tanks and the rest snipers or some other weird setup it would be bad to lay out some boundaries.
So any ideas for it?
Maybe someone as some good statistical source at hand that could show the average allocation of unit classes in the various conflicts, that could be handy as a guide.