I'd recommend 2 things;
1) Get TH's "EZ Water Calculator" from here: http://www.ezwatercalculator.com/
2) Get a water report from your water district if you have city/public water. They're typically required to provide a water analyses report for you (by law).
- or -
Get a sample and send it in for analysis. I'd recommend Ward Labs. Go here: http://www.wardlab.com/FeeSchedule/WaterAnalysis.aspx and get test W-6 "Household Complete Mineral Test" done.
The folks that have responded are correct though, the information really isn't sufficient to give an accurate answer. My guess is, based on your information, that the water changes over the course of the year (possibly due to the area drawing water from more than one source). I say that because, like the others have stated, the values reported span a fairly large range. If your water does come from various sources, then you'll be dealing with different water all year long (PITA). I'm fortunate that Virginia Beach provides a quarterly water analysis report and it remains fairly consistent all year long (indicating a consistent water source and water standards).
Try googling "water analysis report for --insert your town/city name here--". See what comes up