I know a light meter would be better, at the moment I don't have any money for that though so the meter in one of my other cameras would be a great solution for a while.

A light meter works for all cameras so there should be a way to use one camera with a lightmeter and do something with that reading for another one. In case this was impossible there should be different meters for each camera right? I mean the lightmeter would work for the Hasselblad and for the Nikon, so the Nikon should work for the Hasselblad, there is a relation.

But there is no doubt your suggestion is the ideal but for now impossble solution.