I don't see why that matters. They both made predictions based on someone who they had no knowledge of. It's instinct.
But Ginyu's instinct revolved around him assigning base Goku based on his look and his observation over base Goku. Freeza at least experienced Kaioken (if we assume Goku used Kaioken), so he had sightly more grounds to base his assumption off.
Ginyu also tangled in a brief clash against base Goku and concluded his full power would be very close to what it actually was. I believe Freeza tangling with low levels of Kaioken would give more resources for him to evaluate Goku's full power... over concluding that over a technique he hadn't even experienced.
I like the 3M number for base Goku because it's very well known and sets a good baseline afterwards.
I like something like this:
Vegeta : 2,400,000
Freeza [post speed increase] : 3,200,000
Son Goku [post Zenkai] : 3,000,000
But if Goku had 2 base powers, then we'd have to assume Goku was like at 2,700,000 at first, which makes the gaps kinda ugly.
But just looking at the black and white manga, Goku seemed to share some Kaioken traits. He had some white-ish pupils and his aura was more intense, resembling that of a Kaioken. It also makes Freeza's prediction of "my 1/2 power can turn you into cosmic dust" more sensible, as in, if he caught a glimpse of Goku's Kaioken, then he has somewhat of a bigger authority in predicting Goku's full power with higher leveis of Kaioken. Still kind of a stretch and makes Freeza like a God in divining one's power, but a lesser proportion nonetheless.
Assuming Goku was initially at 2,700,000 and that supp. Freeza was 2,800,000 and then FP base Goku was 3,000,000 with no-hands Freeza at like 3,250,000 raises some problems to me during Mirai Trunks's arrival. Following the Daiz scaling, placing Piccolo and Vegeta in the middle 3M territory seemed always a sensible approach. If Vegeta is 3,500,000 and Piccolo is 3,200,000, then that'd leave them at higher than no-hands-Freeza, which I kind of don't like. I like to place Piccolo (mirai trunks debut) at the level Freeza used to trounce Vegeta... the no-hands Freeza territory should still be unreachable by Piccolo and Vegeta imo. Vegeta too. So making Goku using the Kaioken would solve this, with Kaioken Goku and no-hands Freeza being still unreachable by Piccolo and Vegeta almost 2 years later. If we assume no-hands Freeza is like 3,250,000 over his more suppressed power against Veggie (2,800,000 or so), then that places Piccolo and Vegeta almost 2 years later at higher than no-hands Freeza... while assiging Piccolo the level Freeza used to fool Vegeta seems more appropriate, leaving then the no-hands Freeza and the level Goku battles against him as a low level Kaioken, way stroner than mecha saga Vegeta and Piccolo.
