Quick version: resistance is directly related to length of wire used between the terminals. For you, with a protank, this will be the entire length of wire you use to wrap your coil. An absolute must: get a multimeter, or at the very least a
mod capable of reading resistance of the atomizer.
[EDIT] I noticed you were looking for a recommendation on wire gage as well. Given the small amount of space in a protank head and the relatively high resistance you're shooting for, I would recommend 32 awg Kanthal. You can go 34 awg, which will need fewer wraps for that resistance (see below). You can also go 30 for more wraps, but I wonder if it wouldn't be a really tight fit with all that wire. [/EDIT]
tl;dr version:
More specifically, the resistance of your coil depends on the length of wire used and the gage of the wire. The higher the gage number, the smaller the wire, and the more ohms per unit length. Thus, you need fewer wraps of 32 than 28 awg for a given resistance. One technique that many here have used with success is to measure the resistance of your wire with multimeter leads BEFORE wrapping. Place one lead at the cut end of your spool of wire, and move the other along the wire until you reach roughly your desired resistance. Cut the wire at the point where the second lead of the multimeter was when you read your resistance. Use this piece of wire to wrap your coil, wrapping as many times as necessary to leave only enough wire on the legs to reach your terminals.
In general, it takes a few times to get it just right. The length of wire you use will depend on the number of wraps made, the diameter of the tool (or wick) around which the coil is wrapped, and the length of your legs. The good news here is that kanthal is fairly cheap, especially when bought in bulk. So don't be afraid to wrap, install, measure, and start over again until you get it just where you need it.
Welcome to the world of rebuilding, and good luck! We're pic mongers here, so if you wanna show off when you're done, don't be bashful
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