Voltage drop is really just a function of wire resistance for a certain wire size with a certain current through it. Ohm's Law says that V=I x R, so voltage = current x resistance.
So if you have a fairly generic copper wire of a certain gauge, that wire gauge is a fairly standard cross sectional area of copper, and that area has a certain amount of resistance per length (there's charts for resistance of standard gauge at a standard temperature, etc). Larger wire has larger cross sectional area, and less resistance per foot. Anyway, the resistance part of Ohm's law will be the resistance per foot of your wire gauge, times the number of feet.
There are some other things that add resistance, like terminals, etc., but you can fairly well neglect those for a back-of-the napkin calculation like you're doing.
So you know the wire length, and you know the current draw per device, and you know the max voltage drop that you want.
For the ground, you don't need to calculate voltage drop for multiple wire sizes, because you're just dealing with individual devices that happen to all be connected to the same voltage source and the same ground. All the devices are common to that ground wire, so you have a total current (I), a wire length and a resistance per length of wire of that gauge (that gives you the R). If you want less than 5% voltage drop, (V) then becomes 5% of 14.4vdc (or the operating condition you're looking for; engine running, not running, whatever).
Do that first, because then you can include that ground voltage drop in your other calcs (your max acceptable voltage drop is now 5% minus the ground voltage drop).
If you want to calc the voltage drop for each device and each wire size, that's easy too. You have a pump with a certain current on a length of wire of a certain size and length. You have a bilge pump with a different current on another piece of wire of a certain size and length. They're different devices on different wires.
DC voltage drop is much simpler than AC voltage drop, especially when you're going on the conservative side and can ignore the small bits that don't really matter.
You can do the rough calcs, pick the closest wire size and call it good, or go one wire size up for extra margin, or just accept that (for example) 4.1% drop really isn't any better or worse than 5.1% voltage drop because it's a just a boat. Basically, calculate the big important stuff, ignore the rest.
You're not trying to run the smallest wire possible for temperature rise to save weight (racing), or trying to save money by using the least copper possible, etc.
It's really just one formula, applied to each part by itself.
