Rowan,
The colour chart is a good way to check your hydration levels. If your aim's good.
The relatively mild dehydration you've suffered could lead to vitamin/mineral imbalances which leave one feeling really rough. Try mixing 1:1 fruit juice and water. Remember also that drinking too much can lead to flushing of vital nutrients, again leaving one feeling ruddy awful.
Dehydration isn't just a question of drinking, your solid diet also has an effect on how quickly the replacement fluid is absorbed. I'd recommend The Complete Guide to Sports Nutrition, 6th edition (2009), A & C Black, £15.99 written by Anita Bean. She's former athlete and now a qualified dietician. It's a very readable and serious book, the oatbran muffin recipe in the back is pretty good too. Recovering from serious kidney problems I'm paying a lot of attention to this right now.