Richard's formula turns out to be quite accurate. I got 180lb of LR and 90lb of LS and as you can see from the picture, it is plenty full, yet enough room to keep things back and put sponges, coral, etc in front and give the fishes a place to swim without bumpin their noses:lol: I went through the same questions and research as you are and found out the seltzer water works great and does not seem to harm the other things on the LR. You will still probably find some critters you may not want, but dealing with it will be easier once you see it together, as I did. My tank is about 5 1/2 ft to the opening and I have to use one of thse 2-step stools to really get my arm in deep. Invest early in a pair of long tweezers ... they work. I tried the bottle traps, but they are kind of messy when you pull them out to change the bait (at least with the plans that I followed. Spend time up front over a rubbermain dish pan and 2 liter bottles of seltzer and a set of forceps ... you'll be glad you did.
If you go for base, I do not believe you will be as happy. Raising the first half off the floor of the tank might not be a bad idea, cause pulling it out after it's in is not fun. If you see crabs that yu do not want, give it some time. They get bolder and come out further each day. Bait them with krill or shrimp and grab 'em with the tweezers. Or just whack 'em with a hammer :hammer: :crazy1: :fun5: . I might think that a good wet-vac might work too ... :idea: ... lots of suction power ...
Again, I do not believe you will be sorry (maybe forego a few beers or something and get the extra LR.... and there's always some very good surprises with part2.
Good luck and keep us posted ... Bruce