Iron levels should be 0.1 to 1.0 ppm. If I had to recommend a specific ppm, I'd say 0.2, but I just go by plant condition.
Iron has a fairly low toxic threshold compared to most other nutrients, and you can definitely add too much. But the acceptable range still gives you a tenfold margin of error, so toxicity is fairly rare. See the black streaked anubias and discussion on page 2 of this thread
for a possible example.
If iron is too low, entire plants will lose their normal rich colorations and become pale and yellow. Here's a diagnostic chart which includes all major nutrients, including iron:
As for calibrating any test kit, it's just a matter of adding a measured amount of nutrient to a measured amount of distilled water, to produce a known concentration; and then testing to see if the result matches what you expected. The Fertilator
will allow you to calculate the result of any given amount of Flourish Iron and water, but if your test kit's lower limit is really 0.25ppm, then it might not be particularly useful. Plus iron tests are reputed to be highly inaccurate, probably because iron can exist in significantly different forms (EDTA/DPTA chelated, non-chelated, gluconates, etc.)