Well, it's not always just about being clearer. It will taste better, too, whether you leave it for another week or two in the primary or move it to the clearing tank. Young beer tastes ok, but like young beer. Mature beer tastes, well, finished. Better and smoother and cleaner and clearer.
But it's so hard to wait for that! If you wait to bottle, you'll have less sediment in your bottles when you pour. It'll be a cleaner tasting beer without a yeasty bite. It'll be really good. If you bottle sooner (but still after it's done fermenting), it'll be drinkable.
Oh, yeah, to estimate your ABV:
(OG-FG) X .131= approx ABV.
So, say your og was 1.040 and your fg was 1.015 (just to make the math easier for me):
(1.040- 1.015) X.131
25 X .131= 3.28% ABV