Gero's power sensor seems to work just fine for powers much lower than himself. He seems to indicate that Vegeta joining would make the fight more interesting.
Base Vegeta > Piccolo Suppressed
Gero says that if he absorbs the remaining people, minus Vegeta of course, he can tangle with Vegeta.
Gero + Gohan, Tien, Krillin, Piccolo Suppressed = Beating Vegeta
After he absorbs Piccolo, when presented with the possibility of absorbing Piccolo, he offers the same statement, that he could tangle with Vegeta should he add this last piece.
Thus, Gohan + Tien + Krillin ~ Piccolo Suppressed
Here's how I have it...
Piccolo Suppressed 25,000,0000
Gohan 10,000,000
Krillin 8,000,000
Tien 7,000,000
It is not 100% concrete, but it sure is a hell of a lot better than pulling shit out of your ass. It gives us structure in a diifcult area.