Well, for starters I am humbled you sent out a mass thread addressed to me

. I appreciate the vote of confidence.
Regarding your question, I am not aware of a calculation that is a catch all for all machines, I think there are likely more than one variable and maybe different by machine and individual use
IMO, on the minimum side, I think somewheres (my belly rub) between 1/2 and 2/3 the loader lift capacity is a good start. On the maximum side, I think the weight of the properly fitted backhoe (for you machine) is the point to me where going up a steep grade the front end starts getting light (so 3/4 of the lift capacity of even a little more as a max). I think if steering gets light that’s a sign.
So in the case of a B2650 let’s do the math. 534A loader lifts roughly 1146 lbs at pin and 827 lbs at 500mm. My belly rub says you want minimum 500-600lbs depending on what you are doing. My chipper is the best ballast I have and at 900ish lbs, with an empty bucket) the steering is too light to go up a significant grade. For me, I have to be able to get back to the barn with an empty bucket. I would not put over the weight of the chipper on mine because of my terrain. (I have loaded tires as well, but I don’t believe that takes the place of rear ballast, at least not in my case…3pt ballast takes weight off front axle in my understanding.
On b2650, I’d be happy anywhere from 600-800lbs and would be willing to do more if flat terrain. I would not consider a BH on my B due to the terrain at my residence. Anyway I think my belly rub works for me. 2/3 or so of loader lift capacity is a good start point. Then think +/- another hundred pounds or so depending on what doing and the terrain. At the end of the day I think BH is the ultimate rear ballast if you can safely navigate the terrain. (On my MX it feels like a tank and incredible stability and traction…JMHO).
By comparison, on my MX, it’s a 1065 loader lifts approx 1 ton, and I think 1200lbs is a good start but so long as on flat ground I like the BH for ballast which I think is in the 1800ish weight range…although AI seems confident it weighs just 50lbs…but it sure feels like it weighs more than 50lbs.
I’m both my examples, I think my belly rub is close, but it’s not really a standard unit of measure.