I assume you have to convert them into equivalent units. Since they are both measures of the amount of a hormone present in a volume then there should be a way of converting them.

Since I never use these ratios at all then I don't have the specific answer. FT3/rT3 and total T3/rT3 are both regularly looked at by some doctors and so they must be able to convert the units.

I just don't know the maths because I don't use the ratios as I think they are generally unhelpful.

Paul