Somewhat different flow rates but not enough to be a major concern. I have described a way you can get a pretty good estimate with out a lot of work.

I know or cannot remember another member doing this over the years. It is a really simple test that will answer your needs. After some thought the flow rate out the return line should remain pretty much a constant. So a volume test at only a slightly increased idle of the flow from it is enough to establish an overall value. The only figure you will get from examining the lift pump output while open ended is both massive and not accurate at all. For your needs.

I am reluctant to add a lot of other considerations at this time. Until what I have mentioned is well understood by you. It might help if another member confirms what I am suggesting.

The primary message is forget the lift pump in establishing flow volume of these systems. Maybe a better way to describe it is the lift pump while in the actual system. Only pumps as needed. Not constantly at a fixed rate. It spends a lot more time barely pumping at all than anything else. It builds up a fixed pressure and tops it up when that pressure drops. Never rising to a higher pressure. I have never seen the factory designed output pressure specification. Think about thirty to thirty five pounds on a good pump though.