For me personally, when I have done touch ups around bumpers etc, a single piece of mylar goes down.  Same goes if its a small square ie ball drop area from wire ramp.

If its touchups in say the middle of a playfield, I will lay an entire sheet down over the whole thing.

If there is polish/wax on the playfield, this will have to be removed as the mylar wont stick to it and give you more grief.