Circuit pad formula in multi-instrument?

Hi,it seems that multi-instrument can be used to measure about 50V for electronics.
But no circuit pad formula was there in the manual.
By the way,how to do this for an accurate number for testing your power?For example,i need to test only 30V,any formula to calculate your attenuation resistor wiring your soundcard?
Thanks!
But no circuit pad formula was there in the manual.
By the way,how to do this for an accurate number for testing your power?For example,i need to test only 30V,any formula to calculate your attenuation resistor wiring your soundcard?
Thanks!