I usually observer an out of phase measurement of ketones expressed in breath (acetone). I've been doing some interesting experiments with periodic high carb intake and notice that my "ketones" actually go up. I'm reading a bit more and it might be that I'm also ingesting a lot of fat with those carbs and might be creating what are called exogenous ketones - hard to say really I"m still trying to work it out.

I've recorded net carbs and ketones for quite a while you can check here if interested http://downhaul.com/lowcarb/daily.txt

Not sure about mag water, I've used dr. davis' method too. not so much lately, I doubt having high mag levels will allow you to handle extra carbs but who knows. Try it for awhile and see!