Yah, I have been trying to figure this out as well. It is pretty touch and go. I had been pretty steady with the 800 methylfolate and 1000 methyl b12.. this caused some start-up which was mostly fatigue and ickiness. I assumed it was potassium and upped that, taking like 200 mg every couple of hours. It seemed to help a little, but I'm not sure. About a week into the start-up I started having fairly constant tingling. It is not the same as the lack of B12 tingling that felt more like pins and needles, it is more like what is described above.. almost like a fizziness. It does feel like, as Fredd says, that the old damaged nerves are being activated.
So, I experimented a bit just to see if it helped. Last weekend I was going on vacation and decided to back off the Folate and B12 for a few days.. just 2. And sure enough no discernible tingling. Then, I returned home and upped the b12 for a few days to see if that made a difference (thinking maybe it was an insufficiency). And it seems that taking more b12 leads to more tingling and nausea... and headaches. So, I have deduced that it is an activation of nerves AND maybe still a bit of a potassium thing. So, the real question is, is this tingling ever gonna go away?! Or do we just have to keep throwing b12 and folate at it and hope that it doesn't drive us too far down the potassium rabbit hole. Such a slippery slope. I'm exhausted just thinking about it.