OK, so being just a bit geeky, I did an excel spreadsheet that takes the time and height of the two extremes of the tide in question.
I then nominate the height I want and it's gives me the time that height occurs.
The tide height is only calculated on the old 1/12, 2/12/ 3/12 system and interpolated to the height required.
Not sure if the 12th's rule will be accurate enough for what I want.