Bleach bath using box lightener?

I'm out of bleach, but I need to bleach bath to get rid of peachy tones - my hair is mainly very pale blonde/pink but there are some areas underneath and round the back that are darker and have hung on to residual directions cerise/tulip/poppy - I've tried all the other methods, vit-C, coconut oil, dish soap and Jobaz but it's very stubborn

Anyway, a while ago I picked up some Scwartzkopf Blonde Ultime Lightener LXX- which lifts "up to 8 shades" - it was ridiculously cheap like £2. I've used it before neat and was impressed with it compared to my bog standard powder bleach and developer.

Could I just mix it up as normal and then add the same amount of clarifying shampoo and do a bleach bath on those dark bits? Obviously I don't really need to lift 8 shades, more like 2 at the most.

Or is this the worst idea ever? Only because I'm strapped for cash atm and won't be able to buy any more bleach for a while.

Thanks.