From FrugalReef

Select your salt mix, set your salinity and enter your measured levels to calculate how much water you'll have to change to correct the level in 1, 2, 3 or 4 water changes.

Feel free to use this application on your own website, as long as the logo and link are left intact. Download Link.

This calculator uses the data published by http://aquariumwatertesting.com from it's salt mix assay and their recommended levels in order to help you calculate how much much water you would have to change in order to get a wayward parameter back in line. In some cases, of course, it may not be realistic to correct a problem with water changes, and you may have to add supplements, or change routines. The tool will help you make these decisions as well.

Select your salt mix. Enter your Salinity in parts per thousand. Then enter the measured amounts for the parameters you're interested in. Column 1 will contain the amount of water you'd have to change to correct the parameter in one water change, column 2 for two water changes, etc. If these numbers are resonable for you, you can correct your parameter problems with water changes, which is always the best way to correct a parameter when the option is available. If the requirements are too high for you to undertake, you know you will need to take more drastic action.

Of course, you should be aware that salt mixes often change quite a bit from batch to batch! These numbers are just guidelines to help you formulate a plan of attack.