Your English is far better than my Italian.

That's the correct formula for adjusting film speed, but use the following to adjust exposure time:

corrected exposure time in seconds = (t+1)^(1/p)-1

where t is the metered exposure time in seconds and p is the Schwarzschild exponent. I should probably also state clearly that the metered exposure time in this formula is at the declared film speed 'd', not at the relative film speed from your Schwarzschild formula.

Lee