Correct. The "f" numbers are a ratio, as a means of setting exposure they are independant of format size and focal length.
But, using a DSLR as a meter is a kludge at best. Treat yourself to a decent handheld meter, it need not cost more than $50. You won't regret it, I promise you.
I doubt that a decent spot meter can be had for $50 not even $100. With many meters I actually rather guess at the exposure.