If you own a Checkpoint level with a Stubby or Magnum base or a 3D DMS Professional Level, their slope pins can give you exact slopes or grades with no painful calculations.
The neodymium magnets on the base of the mentioned levels hold the flat side of the slope pins precisely in the right position so you’ll get an accurate slope every time. For example, if you need a 3/8″ per foot rise, simply place the 3/8″ slope pin on either the (+) or (-) slope pin location marked on the base and position the horizontal bubble vial to read level.
The metric set comes with 10mm, 20mm, 30mm, and 40mm pins while the imperial set comes with 1/8”, 3/16”, 1/4”, 5/16”, 3/8”, 7/16”, 1/2″ pins. Both come with a case and lanyard and list for $23. You can find either set of slope pins starting at $20 plus slipping, handling, and possibly tax.