When trying on a teamup plant when it is the only one in that lane, what I started doing is hovering over a bit, and then wait incase the plant moves. If the plant moves then I hover still toward the target and when I am "blocking" the plant from letting me take the spot that it is presently in, and approximately half of the target then I place.
Try in random battle with seeing how much play you have, and it helps give you a feeling for what works best.
I know I had trouble at the start, especially with cob cannon....you missed and don't get do destroy the annoying card makes you want to quit. Same thing with admiral navy bean and jelly bean. Can just picture the other player wonder what the heck and then laugh...