Hmmm?
From the pictures, it appears that the Hub Bar is a One off Bensen Clone. It also appears that the Hub bar has a Pre-defined twist for the blades.
The Bensen blades had a Pivot point using a Roll Pin, and with a set of "Tracking bars" could be dialed in to precisely the angle you wanted. On my Bensen blades I would set them at 1 3/4* (I think) My tracking bars are 48" long and had a mark of like 1" at 40". Set up like a scissor, I would set each blade the same and they would spin at about 375 RRPM and fairly smooth . I think 1 1/2* produced 400 rrpm and 2* produced 350 rrpm. I think mine were 21' diameter rotor.
That has been 15-20 years since I made them. Memory is a little fuzzy, but it seems like it was calculating the Cotangent of the angle. I would have to do some study to refresh my memory on that one.
If your Hub bar has a built in twist, then I would assume the angular difference between the blades and hubbar would be "0".
but I would want to measure the angle between the rotational center of the hubbar and the blade mount, and determine the angle. This angular difference is your blades "Angle of Attack". As you increase the angle the blades will fly slower to produce the same lift, in other words, turn slower to produce the lift needed for the same "All up weight".
The Centripetal force created by the Rotors RPM is a vital part of this equation. The Centripetal force is generally referred to as "Beam Strength".
If your blades are a set of "Phenix" like Barnys were. I would think you would want around 350 RRPM.
Keep in mind also, as you decrease that air density by either , elevation or altitude or temperature or the combination of these factors, the blades will automatically spin faster to produce the lift needed for the same "All up weight". Blade pitch can be adjusted to produce the desired rotor rpm.
Another reason to go to El Mirage, those folks fly in that density of 7-9 thousand feet all the time.