How To Level Uneven Fireplace Hearth Concrete?
If there are high points above the desired end level, use a silicon carbide block (you'll find them in the masonry section, with grooves in the face and a big plastic handle on the back - roughly 4x8x2 inches) to grind them down. They work surprisingly well. If the floor you are trying to match is level, then a self-levelling underlayment product (generally a form of gypsum cement) is the typical approach to make the surface level and even. If the floor is not level and you need to match it's non-level-ness 5/16" down you will need to fabricate a screed to run on the floor and level 5/16" below the floor, and use something less self-levelling, like mortar, so it stays put when screeded.