the unknown, i understand,is the exact distance of the iceberg from the Titanic at the time it went hard a starboard at 22 knots.. which is about 38ft a second... and that is because two thirds of the berg is under water - so its submersed size/ diameter could be much bigger than what sits above the water.. if one calculate the time from the call "iceberg right ahead.." to the point of impact, one gets an idea of the distance, and given the Titanic's turning circle of 3860 feet one can plot how far way from the berg the titanic would have to be to avoid something easily as wide and probably wider than itself... i have read this -and it says the berg was about 500 yards away, when sighted.. and as a guide and i cannot certify its accuracy, i just share this..
The Turning Characteristics of the SS Titanic by Samuel Halpern - ppt video online download - so.. 1500 feet wasnt enough to avoid the iceberg, not even its above water part, since ice fell onto the deck.. it took about 37 seconds or perhaps one or two seconds more, to cover that distance, and reach the hidden part of the berg.. Another 25 seconds or 300 yards might have saved it from contact.. as an estimate then i suggest 800 yards at @22 knots.. ie about half a mile would be needed - it is the hidden size of the berg that isnt known, i.e its shape under water..