Risking the integrity of my head

I'm dying my hair right now -- a practice that has had, in the past, drastically irregular results.

The thing is, my hair is naturally an almost-black shade of brown. The color I'm going for is white. So, I need to bleach it to an incredibly pale yellow before it actually works. I've succeeded once, and failed at least twice. 

Today, I'm starting from the position of the lightest hair I've had in a while, I'm trying T-18 white dye, and I'm using level 40 developer. I'm planning to leave it in for an hour but if the burning hurts too much before then I might jump in the shower early.

For reference: the dye recommends level 20 developer, for a half hour. According to the internet, you should only ever use level 40 developer if you're a professional, in a salon. So a plausible worst-case scenario here is that I get to try out the bald look.

Also, I have to go to work an hour and a half after I'll have finished. So, there'll be just about enough time for me to learn how shaving a head works, unless the burning baldness is surprisingly even and clean.

Will post pix later.