Ideal distance between chainring and chain stay
4mm at the closest point is about the safe spot. 3mm is about the sane bare minimum you can go to if you want to push things. Less is asking for trouble. Note that frames, cranks, rings, and spindles do vary in how flexy they are and riders vary in habits and strength, so one can only approximate here. If you stacked the movement from a bunch of flexy things under a strong rider, 4mm probably wouldn't be enough. But it works well as a rule of thumb.

A simple way to actually do this kind of measuring is slipping an allen wrench in the gap, essentially using it as a feeler gauge, or just eyeballing it with one. 2, 2.5, 3, and 4mm wrenches are conveniently sized for the purpose.

It sounds like you're sacrificing an optimal chainline to make use of an 18t cog you already have. I kind of hate that idea; having perfect chainline is one of the big payoffs to all the work you're doing here, and makes fixed/singlespeed bikes work like they should. If this is fixed I would just go for a 14t cog instead. It's true that it will wear out faster, but I think it's worth it. 

With steel, another approach is just to dimple the frame, if the clearance that could be gained by doing so would be enough. It's less of a big deal than it may seem like. Frames get squished in the chainstay area at the factory in ways that aren't terribly sophisticated. Usually techniques for doing this involve making a wood form to cradle the back of the stay while you push something rounded into the front, using a vise or clamp.