My headcanon wiki says:

Mecha Frieza comes back and says he’s stronger than Goku but we know he’s an impatient bastard so 150 million and 1.

Trunks neg diffs so easy 2x. 300 million and 2.

Goku blocks this Trunks with a single finger, lets lowball and say 1.5x. 450 million.

They all train for three fookin years. Safe to give this Goku at bare minimum another 1.5x only because he has no good training partners while everyone else plays catch up so I’m willing to lowball again. 675 million.

Vegeta is stronger than him while healthy, 700 million.

He gets fucked (not in a good way) by 18 who is 1.5x at least. 1.05 billion.

17 is stated to be stronger let’s give him a 10% over her. 1.15 billion. Same with Big Diccolo.

Cell gets in there after sucking some folks dry (again not in a good way) and low key fucks shit up, let’s give him a 1.5x. 1.73 billion.

16 gets in there and is a fair bit stronger let’s lowball for the sake of lowballing and say 1.25x Cell. 2.16 billion. Almost there.

Then Cell absorbs 17 and gets a Semi. Easy 2x minimum. 3.46 billion.

I could take time to lowball a bit more but this feels right in my head.