Glyphosate is not the best choice for kill edging. It bleeds out way past where it was originally applied. The fashion in Hawaii is to make walls out of stacked and mortared volcanic rocks. Almost impossible to trim nicely. Diquat or Finale works well as a kill edger applied in a 1-2" band next to that kind of wall. Do it with RoundUp with a non calibrated hand piece and the dead area could be as much as 6-8" wide in addition to not being a sharp line of kiled grass. An 8001 tip operated at 40 PSI and held close to the ground will apply a 2" band.
They who can give up essential liberty to obtain a little temporary safety, deserve neither liberty nor safety.
Benjamin Franklin 1775
Sell not virtue to purchase wealth, nor Liberty to purchase power.
Benjamin Franklin Poor Richard's Almanac1738