How did you level your floor? My basement floor is also about 1.5"
out of level between high and low spots? Did you use leveling
compound on the entire floor, or did you build a sub-floor using 2x4s
with shims and plywood? I was planning on building the subfloor, not
using self-leveling stuff. The 1.5" seemed way too much to use that
self-leveling compound.

If your basement has enough head room, you may find that building up
the subfloor using 2x4 with shims and plywood to make the floor level
is a good idea. However, my basement room is really
"vertically-challenged". Therefore, I cannot use the 2x4 idea.
Instead, I pour concrete to build up the subfloor, and pour leveling
compound to smoothen the surface.

And yes, you are right to say that 1.5" of leveling compound would be
too much. That would not only cost too much, but also the leveling
compound would run all over the place before we could build it up to
the desired thickness. I built up the floor up to 1/8" or 1/4" below
the desired thickness, and poured leveling compound to reach the final
thickness. By the way, there was a lot of squeezing involved when I
put down the concrete and the leveling compound; that took a lot of
preparation and effort. Therefore, you are probably better off using
the 2x4 idea if your basement have the head room.

Again, I must point out that I am not an expert in this area. I am
just telling you what I did that may or may not be the best way. You
will have to decide yourself.

Hope this helps.

Jay Chan