A YM instructor once told me that it wouldn't be adviseable to use the twelfths rule in the BC, because of the big ranges. So being a contrary bu**er I took about 20 tides from 2007, with a range of springs and neaps, and found some averages.

The twelfths agreed with Belfield, Reeds and WTides to within a few percent on time and height, so I now have a little excel table with ranges from 1m to 16m and the corresponding tide height for each hour.
I used to do all the calcs very carefully when anchoring to find minimum depths etc, but for the last couple of years, having seen what happens on a daily basis compared to prediction I think twelfths are as good as anything. ALL the different tables and programs are inadequate at some point, either the algorithms differ slightly so times/heights don't agree (eg Belfield is very accurate on Springs, and can be way out for LW Neaps), or the conditions on the day may surprise you.

For example, on the last neaps the tide turned an hour ahead of LW prediction, and the tidal heights on the ebb were about 1m below at predicted time. High pressure is said to depress the tide, but that's not always the case.

So if you want to calculate to the minute and centimetre in the BC, I would say don't bother. Using the Rule of Twelfths is sufficient, because IMHO if you plan to leave a few cms under your keel then you are asking for trouble. And if the tide goes against you when trying to get past that tidal gate (ie between Hartland and Sharpness

) it may be that you haven't miscalculated - it's what it has decided to do on the day!