Having automatic dosers is really the only reasonable way you get a vacation of more than a couple days away from an SPS tank.
Putting a sitter in charge of an SPS tank where water changes or dosing will be involved is a dicey, dicey proposition. Even if they are "SPS-aware". If not, I'd trust your cat just about as much as a sitter. I've seen it go wrong so many times that it literally may be better just to have your tank left alone while you're gone. Dim the lights to minimize alkalinity usage a little. Maybe run the tank a little cooler as well...maybe by 2 degrees or so? Fragging back your corals a lot will also reduce consumption.
There's no ideal way to go about leaving an un-automated SPS tank.
-Matt
Ya I agree with you. I might have to show them water changes..
For kalk, do I have to know exactly how fast evaporation occurs and how much?