The problem with saying Demon Mark not complete = Meliodas not at full strength is that your saying that he's weaker here:
Then he is here :
Because the Mark is clearly more full in the 2nd instance then the first.
Seeing as those two instances happened like 5 chapters apart, and he didn't die in between can you explain why would he be more powerful in the 2nd instance then the first?
To me it makes more sense that nakaba is using the mark as a measurement for us to gauge his corruption rather then his power.
He was clearly more evil/angry vs. Fraudrin then Monspiet and Derririe probably because of Liz and danafor, hence slightly more corrupted, hence the mark almost complete.
And it validates fraudrin's comment that what he has now is what he had 3000 years ago with the mark active.
Now there could be a difference in extra power provided between the half filled in form, and the almost complete form, but it's impossible to tell because all his opponents were too weak.
But however strong he is I hope nobody's expecting him to get another like 50k increase, when it's complete if he's still not at full strength.
Whatever he showed vs Fraudrin was clearly just short of it, if not all of it, with the emotion stealing just not fully complete so the king can be revived.