Steve, the directions from the Great Yellow Father (Kodak) indicate the same processing times for their 120 as their 35mm films of the same specifications. However, 120 negatives seem to accept a slightly heavier exposure and still print well.(posted 9342 days ago)To determine actual film speeds, you would probably need to shoot a series of identically exposed films and them process each of the films at say, as recommended, 10% more, 20% more, and so on.And then use a densitometer and lots of scientific junk. You need not be that pickey to find settings that work well for you.
Regardless, actual film speed to give a set tonal range and reproduce a 18% grey accurately at its film speed are conditions of the film.
What might be happening here is that your Hasselblad shutter speeds at any number are not identical to your 35mm camera...that the 35mm is giving "longer" exposures at its settings than the 'Blad. Or the aperatures of your 35mm lenses are not stopping down to exactly the same settings (though the same numbers) as your 'blad lenses.
Have fun, experiment, test. Jim