The Dept Alternate issue has always been fuzzy,more so now with transport types doing single eng auto lands!!the original concernwas having some place to go if thhe WX was too Low to return,ergo the closest'open'field was chosen.In some cases this might be quite distant in the case of widespread fog....So the powers that be said in view of engine reliabiliy one could go quite far on one..Nowadays the A320 can 'plan' an Alternate within 340kms Different types varied distances etc...Flying light twins out of vancouver years ago we would select the closest 'open' airport(usually within 30 miles).
thats 340 nautical miles not kms,,,,,