I would like to make some tweaks on specific pictures manually.
The only thing that helps is "Match Total Exposures" in LR.
Can you tell me how to calculate the Mean of a picture roughly? Thanks.

I'm sorry. Those are LRTimelapse internal calculations that I won't share publicly, I hope you understand that.
And I don't understand either what you are trying to do. There is absolutely no need for "Match Total Exposure" or any other manual adjustments anymore, since you can use the Visual Deflicker to smoothen any undesired flicker effect at the end of the workflow.

I understand that. I am trying to recover a failed HG sequence (mostly because I shoot in Av mode wrongly). There are just some jumps left after processing.

You can apply Visual Deflicker as often as you want with the "refine" option, that should help to eliminate the last residual flicker.