When it's a problem?
How many mm deep the recess has to be to influence the board feeling?
Usually, how you feel the board when it happen?
How to sort the problem out (refill and sanding or just sending to tail end) ?
How to be sure the tail kick is not part of the problem (if there's some) ?
Rocker "flat" aloows any tolerance?
If the negative sits between 30 cm and 60 cm from tasil end, how to refil it when there are hull concaves??

Not worth dealing with unless you are slower than others once fully powered and in the straps.
Negative allows you to plane up sooner than others.
Fairing compound is a mix of epoxy resin and micro balloons/dust, available at marine supply or surfboard supply stores.
Sand with 100 to rougher and clean bottom, add a slurry of compound, allow to fully set, sand with big block, and 3' lo g straight edge until fair, then paint spray a seal cost to keep thin spots adhered.

Think I agree with Lee here, it's only an issue when speed sailing, it can increase the wetted area by pushing the nose down. If you want to go faster, fill it.
As for how much negative is an issue, I guess that depends how fast you are going. We're looking at lift behind the rider pushing nose down. So this lift will increase with speed and amount of negative. If you want 50kts I imagine 1mm would make a difference.
Another thought, this will also depend on water state, the flatter the water, the slower it will make you. For going fast on dead flat water, you need more angle of attack, for choppy water the board needs less angle of attack. You want to fly over the chop, not slam into it.

Filled in negative rocker on my 47w speed board the other day. I had 2-3mm negative in front of my fin but I've still managed 43kts with that. Thought I'd give it a go more out of curiosity than anything. I haven't had a 40kt day since, but have hit 38. To be honest, I couldn't feel any difference, I reckon boom height and mast foot position will make the most difference. Of course if I get a PB in the next couple of months I'll be singing praise of dealing with negative rocker
my advice if your board feels sticky: boom up, mast foot back

I'm definitely a rank amateur with board repairs, so take the following with a grain of salt.
I used an epoxy resin and q-cell bog.
after sanding the affected area I masked it off and applied bog.
I used a level to screed across the bog, using the tape as my level.
I only screed parallel to the length of the board (you can see the lines). Doing it this way any v or concave is maintained
sand and paint after. It seemed to work fairly well despite my given my limited talent