If you have a fridge big enough to put it in when its done fermenting, it can really help clear the yeast. It's called Cold Crashing and it helps the yeast flocculate. English yeasts are usually good flocculators anyway but if you can cold crash it for a couple days prior to bottling it will help get it clearer. There will still be more than enough yeast left in the beer to bottle condition.
It will increase depends on how rapid your boil is, evaporation rate etc to determine but likely 3 to 5 points at least. If it's not what you wanted, you can either add DMS (I should have mentioned to have some handy for this eventuality), or boil until you hit your gravity and live with the volume of wort you've got.
What yeast are you using? I'm guessing the chico strait (US-05, WLP001 or WY1056). It attenuate pretty well so if you don't start at 1.035 or so it's gonna be a pretty lite beer.