Where are all the tile guy's around here??
The issue with adding more water is....the directions specifically say, do not add more water. So what to do??
Contact the company in an e-mail, they may have a boilerplate answer for everyone. Better yet, go to a local tile store, one that knows more than the box stores. Ask them what the installers are saying. Tile installers usually will grip right to the supply house with any issues that come up.
In my experience, a little more water just takes a little more time to set, but sometimes it also lets loose earlier than expected.
So talk some more to the folks who use it, or take a chance with what you are doing. When the tile pops...reset it again. Not a big issue for a DIY er.
Tell us what you come up with.