You could durn near use some mason's line and a clip-on bubble level for that short of a distance. If you set some stakes once you find the grade you should be able to get real close.
Just remember to strrreeeeetch that line tight!

"Forget pounds and ounces, I'm figuring displacement!"

If we accept that: MBG(+)FGSF(=)HBG(F1)
And we surmise that: BG(>)HBG(F1) while GSF(<)HBG(F1)
Would it hold true that: HBG(F1)(+)AM500(x)q.d.(=)1.5lbGRWT?
PB answer: It depends.