How to repeatedly lighten for balayage?

I wouldn't use 40 vol repeatedly, even with olaplex. Certainly not once a week anyway.

You won't know how much lift you'll get in one go until you do a strand test. This is important because I get to a pale yellow from a very dark brown, around a level 2 as well, with 30 vol, a good bleach and olaplex in one session. I have very little red in my hair, but the yellow is resistant. All 40 vol does is lift faster, not more.

If you do it over several sessions, you don't have to paint each hair strand exactly each time. It'll add to the affect if there's some hair lightened more than others. The look is supposed to be a more natural highlighted look, right? The point is the ends of the hair gradually get lighter because they've been lightened more over time.

However, if you go in with a strong bleach and high vol peroxide in the first place, you'll only be doing it once anyway.

I read that you need to bump up a level when you use olaplex, so the 40 vol is essentially 30. Do you think I should still go down to 30?

I watched Guy Tang's videos, and he says to go low and slow. With 30, how long did you leave it in Janineb?

I'm scared to do it more than once because I don't want to miss any parts and have them stay brassy. I fear my hair will have some gray parts and some orange parts that are too small to target...

Yes, they do claim that. I use 20 vol when not using olaplex. I'm still not totally convinced it's nessecery, but as it's cold here at the moment I need the extra boost.

I leave it around 40-50 mins, but it's different for everyone, again where strand tests are important.

I've never really seen it done over several sessions anyway. I don't really understand where that idea came from. But the extra lightness at the bottom tends to come from having it done several over many months. That way it blends.

Unless you are experienced with bleach and lightening I wouldn't recommend trying a difficult advanced technique like Balayage on yourself. It is quite possible that you will end up with a patchy brassy mess. Sometimes it is necessary to have a professional do it, and it is worth investing the money in a really good job.