I've decided that Freeza's initial lunge at Goku was around his "initial" level and that when firing the death beam barrage he's trying as hard as he did when he actually started beating up Vegeta.
Against Vegeta, Freeza ups his speed after a single dodge:
It took Vegeta keeping up with a single attack for Freeza to decide to adjust.
Against Goku, Freeza attacks first, and we can surmise that, with no knowledge of who Goku is, he's only trying as hard as he did against Piccolo and co. (It doesn't actually
have to be exactly this level, imo, but that's not important; it's most likely below what he was just dishing out onto Vegeta is the point).
First of all, it's worth noting that Goku's kick is better than anything Vegeta managed to do, even if Freeza is only at his "initial" level. When Freeza dodges Vegeta's first attack, he's probably still at his "initial" level -- the line "
I get just a little serious and raise my speed, and it seems that you can’t keep up with me" suggests that there was not yet any speed increase when Vegeta
was still able to follow him. Even if Freeza was on the offensive instead of on the defensive against Goku, Goku landing a kick puts him solidly above Vegeta, and Freeza knows this:
He's amused that someone's finally put a bruise on him, even if he doesn't quite grasp the full extent of Goku's prowess.
If all it took was Vegeta following Freeza with his eyes for Freeza to try harder, then why wouldn't Freeza try harder after getting kicked in the face? He should, at the minimum, be trying as hard as he did when he tortured Vegeta imo. Any less than that makes no sense.
Of course, after Goku knocks back all the death beams with a hand, Freeza expresses his surprise at this feat, and Vegeta warns him not to take Goku lightly. Freeza should have already understood that Goku was better than Vegeta after the first blow, but now he knows that there's even more where that came from. (If we get really into the nitty gritty like I told GSM we could do, then you could also totally say that Goku blocking the death beams > Goku's kick, but that's not really important to the overall scaling. I'd like to think that I'm just covering the important levels here.)
Freeza obviously heeds Vegeta's warning (partially; the Super Saiyan shit is still nonsense to him, of course) and fights Goku more seriously, and they're about even. This necessitates a fourth level for Freeza, one that's in-between his "vs Vegeta" level and his full power (at his current suppressed state).
Some quick numbers, I didn't think about them too much but they're just to chew on for now:
Vegeta: 5,000,000
Initial Freeza: 5,500,000
Vegeta's blast: 7,500,000
Freeza (vs Vegeta): 7,500,000
Goku (warming up): 8,500,000
Freeza (vs Goku, warming up): 9,000,000
Goku (full power): 10,000,000
Freeza (full effort, blocking Vegeta's blast, "no hands"): 11,500,000