Peter - Yes, it's hard to know what the % really represents. I find it very unlikely that things would be bigger on your monitor than real life however, unless your monitor was gigantic.
If we knew the actual fov the % represented, and at which direction it works (vertical? Horizontal?) we could calculate exactly what it should be set to to get a 100% accurate size of objects. I.e. if one has a monitor like my own 16:10 24" sitting less than a meter away, then perhaps it takes up 38 degrees of my view (horizontal), and then if one can set the game to 38, it will match exactly.
Abdul - I'm quite happy to see you tried something along those lines and liked it
I'm just really into immersion and have learned much about it over the years, and sharing is fun.