You can use a level on top an 8 ft.-long 2x4 to determine the slope for yard and garden projects. If you need a slope of 1/4 in. per foot, tack a 2-in. block under the 2x4 on the down- slope end. When the bubble is level, you will have 1/4 in. per foot (2 in. divided by 8 ft.). You can vary the block height for other slopes. If you want a slope of 1/2 in. per foot, make the block 4 in. high.
There’s a solution for dealing with wood screws which have broken off below the surface of the wood. If you don’t want to risk damaging the wood by digging it out, try driving the screw deeper into the wood with a nail set. Then just fill the hole with a wood filler, and drive a new screw next to the broken one.