Yet again Thank You for assisting my tilting at the wind blower.
I was planning to attempt an average reading of the return intake... But before that, the filter screen must be cleaned.
...The blower wheel has a surprising amount of build-up dust!! I clean the screen every two weeks. That will have to wait until AC cooling season so after washing the condensate can rinse.
For what little it may be worth... I took some basic measurements:
Return opening 600MM x 152MM
The Kestrel vane anemometer .5 M/S & 105 CFM
Testo Hotwire .45 M/S & 95 CFM The Testo displays cubic meters/H so I multiplied by .59 for CFM.
Next will be the cardboard ducting.
Not sure about air, but have some experience with water flow and usually a flow reading near an elbow or pump is to be avoided usually by a specified number of diameters. Also, I can view the meter face-on without jamming my head against the ceiling.
The blower wheel appears to be 24" end to end NOT INCLUDING THE SHAFT STUB ENDS.
The diameter was a bit less obvious because the coil wraps ~180 degrees around the blower wheel. But by probing and marking the probe; a SWAG measurement of 3 5/8"OD The vanes appear to be ~1/2" deep. This matches fan wheels for replacement 90.8mm(3.57" x 602mm 23.7") ---I could not match any part numbers.
My belief is that most components from China can be second-sourced.
Photo of blower wheel for my unit. 
https://www.highseer.com/products/indoor-blower-wheel-11 Thank you
Ah of course…. This has a crossflow type of fan in it (also called a tangential blower). Since we virtually never see these in the industrial world, I’m not as familiar with these. A company called Ziehl-abegg out of Germany is one of the most well known companies for making this type of fan. I did a quick search and found a catalogue for their fans and if you go to say Page 30, you will find the performance curve for a fan that sort of matches the size you mentioned… https://www.ziehl-abegg.com/fileadmin/Downloadcenter_NEW/00_englisch%28EN%29_MASTER/X02_Catalogues/ZIEHL-ABEGG-Catalogue-Cross-flow-fans-english.pdf Well, at least the blower there is about your length but it doesn’t exactly say what the impeller diameter is. However, scaling off their provided diagram (bottom left of the three diagrams), it looks to be about what you have….diameter of 90.8mm (3.57”). The length of the one on page 30 is 680mm which is a bit longer than what you have 602mm (23.7”). Since the lengths are a bit different, I’d just ratio the flow capacity they show on their curve when drawing up your own curve i.e. multiply by 602/680. As for the diameter, that gets much trickier and beyond the scope of what is possible here. Hopefully you’ll find it to be close when you try to scale it.
The more difficult test parameter is the speed. You can see on performance data on page 30 that they show curves for speeds from 740 to 1,500 rpm. How are you going to get the speed of your unit? If all you want is capacity, you don’t need any of this… the curve is only useful if you are trying to plot the data to see if what you have works somewhat according to design. If you want speed, you’re going to have to get some equipment… a strobe or something that can read off a piece of reflective tape. In a pinch, I’ve also done it using vibration data (that needs some interpretation) but that is trickier.
Will the performance of your fan be the same as the Zielhl-abegg given that they are different manufacturers? Nope… but hopefully it won’t be that far off.
You are correct about measuring near elbows… it’s the same for air. This is why I would make the cardboard about 2 feet long (assuming you can at least do that long) and put holes closer to your return air inlet. You would like the flow to travel at least 1 ½ foot to give it a chance to straighten out before it gets to the point where you measure it. The holes obviously need to be big enough to allow you to insert your vane anemometer…. and make sure you put some duct tape over the holes you aren’t using so that the results won’t be distorted.
I only looked in the Z-A catalogue for a minute and if you have time to browse around it, you might find something that is closer to exactly what you and I suspect you will find this to be very useful to your testing. I have to leave you here as I’m traveling shortly and away until Monday. I will check into FR from time to time though… I am curious to hear how you made out.