I wouldn't worry about the slope as long as it is flat. Building case goods you use a square on your assembly table, not a level. Every tool is square to the table, not the floor. In the off chance you need to check level, you can use an inclinometer such as a wixley and zero it out on your bench/tool. I did level out my first shop by laying out progressive stringers - PITA- 3 shops later never bothered with it and no issues. Did install a dricore clone and highly recommend it.