This is the average x-ray output (μGy/mAs at 40") measured on ~40 radiographic units (fixed and portable units) over about 3 years for both large and small focal spots.
Textbooks say the radiation output should be proportional to kV2. Fitting a power law curve to the data gives the equation
Output (μGy/mAs) = 0.008373*kV1.983
which is pretty darn close to the expected square law.
There's not as much variation as might be expected. Armed with this data, I think I'd be comfortable calculating exposure estimates for patients or image receptors for any radiographic unit.