Well, probably pretty good!  However, just remember that the majority of tide tables and on line predictions are mathematical models and, although generally accurate may be slightly out.  One of the biggest factors that can account for a difference between that calculated (or obtained online) and the real world is atmospheric pressure.  The 'weight' of the air pressing down on the sea can make a difference to how high our tide is.  Most tables are based on the 'International Standard Atmosphere' sea level pressure of 1013mb.  If the pressure is higher, the the sea levels will be lower, and conversely, low pressure leads to higher high tides.  The rough rule of thumb is that 1mb = 1cm of difference.  As an example, last week, we had a pressure, quite low, of 991mb.  that is 22mb below 1013mb, so, in theory, we should see higher tides of about 22cm, or 0.22m.  The tide gauge data below from Portsmouth shows a pretty good correlation!  We love meteorology here at Apogee... if you would like to know more - Get in Touch!