Only have 20 and 40 developer...

Hi there!
Backstory: I have short short hair, like I wouldn't even consider it pixie status yet (though some people may say it's a short pixie). I've been growing it out from a buzz cut for 7 months, and have been bleaching/toning it platinum about once per month for the same amount of time. I'm keeping it shorter on the sides and back, sort of like an undercut, so by the time a month goes by I'm blonde up top and my natural dark brown on sides/back. As suggest by my local Sally's associate, I've been using 30 developer for the Loreal Quick Blue Extra Strength bleach powder, and 20 developer for the Wella T18 toner (lightest ash blonde). I'm getting bored of the platinum thing, so this time I bought Mystic Divine 8NG toner for "natural golden blonde". It came with a free developer, and because I had a huge bottle of 20 at home already she suggested I buy 40 because they were out of 30.
So here's my question: Is it better if I use the 20 or 40 for the bleach, and is it better if I use the 20 or 40 for the toner? I have enough of either for bleach and toner if need be. Last time I accidentally used 20 for bleach so I used the last of my 30 for toner, and I was not happy with the results (more yellow than normal; I've been using Matrix So Silver shampoo the last 7 months, and even that didn't help). With that said, my thought is the yellow the bleach left my hair using 20 may not be a bad thing if I'm trying to go golden instead of white this time around though?
Your help is hugely appreciated!