You can't make an 18mg shortfill with 72mg nicotine base without diluting the flavour. The maximum you can make without dilution is 12mg.
A standard shortfill with 10ml of nicotine base is 83.333% in flavour and 16.666% in base.
Making an 18mg mix out of a standard 50ml shortfill with 72mg nicotine base will dilute the flavour to 75%.
You need 107.95mg of nicotine base to turn a standard shortfill into 18mg without dilution.
You can get this (and much stronger) from abroad, but it's harder, riskier and illegal.
You'd be much better looking at longfills and bottle-shots if flavour matters to you.