Gotcha. I do that when I'm in a rush myself, and call it a ghost spray. It works, there are no problems with it, but I'll use the 1.5%-ish solution and just apply less.

Denser solutions can cause some burning in marginal weather, so be forewarned. The dots of ferrous sulfate sit on the blade, concentrate as the water evaporates off, and the acid burns the leaf. You'd be better off doing less more often.