Jason,
First off, thanks for such a great product.
The ideal range for each test is a great idea. It would probably be best to:
1) Locate the range right below the test in the first column. Might need to make it slightly wider.
2) Provide a link or drop down to post standard ranges for various typical pool configurations.
3) Allow the ranges to be customizable since you won't be able to cover all the configurations out there and some people operate within customized ranges.
4) Have the data saved in the cookie like gallons.
Another idea, but it might be too hard to implement, is to provide an order of adjustment for the person that has to adjust several aspects of their chemistry. For example: FC is low, PH is high, TA is low, CH is in the band, CYA is high, Borate is N/A, and Salt is low. The newbie might start at the top and work his/her way down the list and waste chemicals added when they try to lower CYA. I know the practical person would figure out a good order to adjust the chemistry, but the poolcalculator is so good now that you are just polishing the cannon ball at this point - and this could possibly help some people out there.
/z