Totally agree with Steve on this one. Many drywallers/painters may give you exact mixing ratios, but that wouldn't be of much help for two reasons. One is because they may be using a different brand, and second is because some mixtures have more or less water content in them, that's mainly due to the variance of the time either product has been sitting on the shelf at the store.
I do like Steve, I add close to 1 qt to a 4-5 gallon tub of jnt cmpd. Usually the indicator I look for is that when you tip the bucket upside down, as to pour into your texture gun, it should "pour". If it doesn't, then it's too thick. I can't over emphasize Steve's point enough though, don't add too much water. Good luck sir.