how much juice would a laptop need?

Yes but.... Keep in mind that a laptop is designed to run on its internal battery. If you plug in the charger/ powerpack then it keeps the internal battery fully charged while you are using it.
There is no fixed current draw with a laptop that is 'doing' something. It all depends wether the hard drive runs, the CD drive runs, the fan runs (and at what speed). How much number crunching it is doing, etc.
The only time you get a fixed current is when the laptop goes to 'sleep' and the screen goes blank.
I have connected my laptop's power pack via an ampmeter and watched the current draw while I used it. It's up and down like a jojo. Wanted to know how long I could run it on my boat. Definitely not full time :-)

If you want to run a laptop from a 12V battery for a long time, check how long it runs on its internal battery. The longer that says, the less total current draw the machine has. The better ones (read expensive) run 5 hours or more on the internal battery. Mine only goes 2 hours :-(
Klaus

OK, here goes; Just did some checks on my old HP Inspiron 2100 (Pentium 233MMx). This computer uses a 19v 3.1A mains power pack.
In the vehicle, I run it off a DC-DC converter putting out about 19.5v, capable of delivering 3.5A.
With no load, the converter draws 45mA from the 12.7v vehicle supply. Charging the computer battery, computer running, the draw from the 12.7v supply is 4A. Once charged, the results are:
- Hibernate mode: 0.2A
- Running, screen shut down: 0.7A
- Running, screen active: 1.2A
- Running, accessing hard drive: 2.0A
These are all current draw at 12.7v. Different laptops will give different current draw, but this should give you an idea. A modern Pentium-M/Centrino laptop will give much better results.
You'll note that the converter draws bugger-all with no load. Running a combined 12/240v inverter and a power pack will not come within cooee of this efficiency. (I could run some tests, but I don't see the point)
Hope this info is of use to you.
Gerry

you said:
"i know conventional starting batts have a percentage of charge that you shouldn't use... do deep cycle batts have a similar percentage? if so, what is it?"

My answer is yes, deep cycle should, as a rule, never be discharged below 50%, they last much longer still if not discharged below 75% or so.
Having said that, if you do discharge them much lower occasionally and IMMEDIATELY recharge them you might get away with it, depending on circumstances and battery brand/ type/ etc.
They are too expensive not to take care of 'em, flattening them too often shortens their useful life drastically.
Klaus