Where do you draw the line between using your trimmer and your 21" trim mower? What I mean is, how big of an area do you have to trim before the trimmer doesn't do the job? Couple square feet? More? I've got one day a week where I might be able to get away without even bringing the 21. I always bring it, and usually use it (depending on how close I get with the WB), but I'd just as soon leave it behind. Thoughts?