Blue-White is the brand I have, a 2-20 gpm variable area rotameter. Info shows maximum 2 psi pressure drop = 4.6' potential maximum head increase. If you have a circ with little gpm change within your calculated head +/- 3', then the flowmeter should work quite well to provide an accurate reading. If the flow chart shows quite a change in gpm at +/- 3' calculated head change, then the flowmeter really won't provide very accurate info, or you need to estimate an adjustment accordingly.
Also, to be accurate, you need a vertical pipe run and quite a long straight distance before and after the flowmeter, i.e, mount the flowmeter midway between a floor to ceiling pipe run. Further, I would mount it on a return line, as high supply temps likely will be higher than the rating of the flowmeter.
I would install the flowmeter with a straight bypass pipe and use ball valves both to isolate the flowmeter and to shut-off the bypass line. That way, you can do your measurements and experiments with the flowmeter, and then when done, shut-off flow to the flowmeter and use the bypass line to obtain the lower pump head.
If you are patient or lucky, you can get a good deal on ebay or similar; otherwise these things are quite pricey.
I would be cautious about getting a spring loaded flowmeter without carefully understanding the specs as to how it might work in a circulator system.