You NEED to pay for an engineering/survey driveway design
They make cheap hand held levels, like a small telescope, about 10” long. Usually no magnification. I would put lath in every so often, say 25 feet measured in, and start at the bottom and work your way up the hill. You will need to know how high your eye is and make a mark on the lath ahead of you with a helper and measure how high the mark is. .
I think there is a misconception of how a GPS device/app determines elevations...
All a GPS can do is determine geographical location...for elevations it must use third party data (nothing to do with GPS) i.e, data from topographical mapping that is correlated and interpolated...That is...once a location is determined it (device/app etc.) then relates that position to known elevations.
If you had 2000 feet of road a hand level woukd get old in a hurry but you do not have that much. Like already mentioned you can check a hand level. Google “two pegs test for a level”.Thanks Dodgeman. I had one of the hand held - telescope levels which I used a number of years ago on the property but it broke (rattles inside and no level indicator any more). But it did work well enough and would work for what we want to do now. I will find one of those. It should be adequate for our preliminary tasks right now - the slope is not great and the distance is not that long so spending half a day doing this will be useful and a bit of fun - I always like learning. Ultimately the grading contractor will be shooting this - we just have time now to entertain ourselves.