concrete question

I have a solid tapering slab that I want to even up. It was designed to taper, as it is an outside sleeping porch (under the roofline though) and it has a drain on one side. I want to raise it and frame it in. SO my question is this:
I'm thinking of putting down about 1 cu yd of concrete (2 inch average over 150 feet) to even it up. However, I need to taper from 3 inches to 0. If I put down a filler layer of concrete and then self leveler, first roughing the surface and use a bonding compound, how thin can I leave the taper on the edges? One inch? two inches?
I then intend to even up the whole thing by pouring self leveler over the top. however, 1 cu yd of self leveler is way too expensive.
Any suggestions?
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