This is how I always saw Gero's calculations:
Goku's projected power: 70,000
W/Kaio-Ken: 140,000>210,000>280,000
Highest Possible Level: 800,000
Gero said they weren't expecting Goku to have any huge power-ups anymore, so the projected power even seems like a bit much. I just put it there for simplicities sake.
What Gero's power radar read when he transformed:
Super Saiyan Goku: 70,000,000
Gero said it was a considerable power-up that surpassed his numerical figures, but I don't believe it calculated Goku's power level accurately. That's why he seemed so confident in #19's ability to defeat Goku. I believe it generated a random level that was way below its actual level. When Vegeta transformed, he didn't think he'd do any better than Goku.
Similar to what BPP said about Gero being shocked at Goku's first attack, he has the same exact expression when Vegeta knocks #19 down with his first attack:
The scientist was clueless as fuck.