Correct concrete mixture/ratio for levelling basement floor
I'm going to be leveling an approx 300 sq feet section of my basement slab in my 1920s 2 story. Finished surface will be cork plank on vapour barrier.
My initial plan was to get 30 odd bags of concrete pre-mix from the big box store, rough level it to within a half inch or so, and then use self-leveler for the surface. I'm rethinking this, as self-leveller is pricey, and the premix is meant for jobs 2" or deeper, so I'd have a large portion of the floor that's 0-1" out of level to do with the SLC.
I'm thinking it might be better to mix my own sand + portland mixture for the top layer (5:1?), sort of a poor man's SLC, but sand isn't any cheaper than premix, at least at my local Home Depot (Toronto, Canada).
Is this mix common? I'm thinking I could use premix for the deeper parts and switch to a sand mix for the top inch or so, then fine tune with a smaller amount of leveler.