I recently did a Oly. distance tri. I felt I did OK on the swimm and bike. But on the run....(I stink). I lost 7lbs, and was told I was deyhgrated. Any geneeral rules... was thinking for doing a 1/2 IM, and if I am bonking on a Oly.
Losing 7 lbs during a race or workout is way too much.
Try this, weigh yourself and then head out for a 1 hour run. Bring a water bottle that you know the capacity of, say 20 oz., for example (fill it with whatever you usually train/race with) Try to drink most or all of it during your run, what you do not drink you can re-measure when you get home to find out exactly how much you drank during the workout.
Then, weigh yourself after the run. Ideally you should not have lost any weight or very little (1/2 - 1 lb.) and you should not gain weight. If you gain weight then you are consuming too much.
This will give you an idea of your sweat rate. Usually 16-20 oz. per hour is a good figure to start with and you can adjust from there based on your own personal trial and error. The temperature will have to be considered as well, on very hot days you will need a little more, etc.