@Wormbo: Not quite. It depends on how games are clipped. E.g. if you have a 4:3 game with a certain vertical FoV angle and a certain horizontal FoV angle and in widescreen modes it adds to the sides you have to set the game to a specific angle so nothing is changed except you add the stripes left and right that your new monitor now has, so to speak.
That's why people figure out ridiculously specific numbers like "106.2602047".
In the end you are right about this being a totally subjective thing though.
@op: If you want to get mathematical you should measure the angle between the left end of your monitor, your nose bridge and the right end of the monitor. Setting the FoV to that angle should more or less make you see the world as big as you would see it if you stood in it.
Of course this drastically reduces the situational awareness you would have actually standing in it, because you can't see anything to the sides of your monitor, which is why games usually pick a much higher FoV than that (at the cost of making everything smaller, which they often compensate for with some sort of zoom mode, e.g. magically zooming iron sights).
In the end you have to decide yourself what you find most comfortable, which is hard (or easy), because you can get used to almost anything reasonable anyway (e.g. between 70 and 110. Lower feels zoomed in, higher feels fish-eyey, although, if you've been playing on 110 for a while 70 is going to feel zoomedin too, of course). Just pick something the game let's you chose and forget about it, is my advice.