Absolutely Chuck, I/rev is easy to deal with 2/rev is the real demon.
I also have a two blade helicopter and balancing equipment, I am extremely fussy about stick shake so have played lots and understand the difference. Short of having perfect track there's not a lot a balancer will help with two per rev.
With a Bensen style layout where the mast points about in the same direction as the rotor thrust vector, I’d guess 2.5” with an 0.083” wall. But that’s only an off the top of my head guess; the calculation isn’t difficult but it takes a bit of time. For Eurotub style gyros where the mast leans forward, it’s probably best to avoid flexible masts and go to some kind of flexible rotorhead mount.
Wolfie, say the gyro weighs 1.000 lbs and the vertical mast is 4 feet long from the last brace, normally the top engine mount. The rotor thrust line is tilted toward the rear at about a 10 degree angle. The bending moment arm is then: (tan 10 degrees) x 48 inches = 8.5 inches. The bending moment is then: 1000 lb x 8.5 inches = 8,500 inch/lb = 708 ft/lb. Takes a pretty stiff piece of pipe not to bend.
That’s the reason it’s best not to stray too far from Bensen.