The problem is not so much that the ounces are different - though they are - but that the pints are WAY different!

The most authoritative way to deal with these types of issues is to covert to a universally agreed upon standard - the metric system - as a go between.

The U.S. fluid ounce is 29.57 cc
The U.S. pint is 473.1 cc (16 U.S. fluid oz)

The British fluid ounce is 28.41 cc (3.9% SMALLER than the U.S. fl. oz.)
The British pint is 568.2 cc (20 Br. fl. oz.) (20% LARGER than the U.S. pint)

So if you just interchange U.S. and British ounces, you are reasonably close - only 4% off. And if you add one more British fl. oz. for every 25 U.S. fl. oz. you are almost dead on.

So, given the most referred to rule of thumb around here:

In U.S. units:

64 fluid oz + 8 fl oz for every 25 lbs of excess weight.

In British units:

66.6 fluid oz + 8.33 fl oz for every 25 lbs of excess weight.

or

3 and 1/3 pints + 1 pint for every 60 lbs of excess weight.

or

3 and 1/3 pints + 1 pint for every 27.2 kg excess weight.

The "British Rule":

For convenience sake, this can probably be simplified to

3 pints + 1 pint for every 25kg of excess weight.

So how do the two rules compare?

At your ideal weight, the British Rule I have offered will have you drinking 57.6 U.S. fl. oz.. Close enough for anything we're doing - your ability to know how much water you've actually consumed as an uncertainty greater than this. Yes, its is a little low, but probably not enough to cause problems. More to the point, the nature of this forum means that few of us are using the rule at this end - we aren't anywhere near ideal weight. And you can always just add a half pint.

At 221 lb overweight (100 kg) they are equal.