...i have this info written on the back of my lamenated Flag Recognition chart. i have no idea who it came from but it did come from this web site, this forum.
Visual Range.
(Him Spotting Me)
Clear Day: 10-15 km.
Rough Seas, Good Visability: 8-10km
Rough Seas, Bad Visability: 2-3km.
"make sure im barely able to see him
whilst flanking."
this is how its written on the back of my flag recog. chart. ive no idea how realistic it is or, if indeed, it can actually be applied in-game?
i have found now and again that i can get pretty close to a merchant on a clear day and for him not to go into evasive action, within a 1000meters?
though sometimes i will go free-cam and zip over to the merchant and see if i can see myself, if i cant i'll take a distance reading on my map and go with that distance untill i forget

(...middle age and all that)
but surely, in game, if i can see myself from the merchant i must be too close. but it is surprising how quickly you dissapear. i'll sometimes do this with my son - who's eyes are much better than mine. "look away" i say to him, then i free-cam out to the merchant - better results in bad weather! - it's surprising how even he can not spot my boat sometimes!!
...or his own for that matter!