It seems to be generally agreed that there are two main reasons for iron stains re-occurring, one is a pH above 7.4 and the other is an insufficient reserve of sequestrant.
In the case of Jack's Magic Purple Stuff, a phosphonic acid based sequestrant, the minimum would be 10 ppm, however I have found the greatest success when I have 40 ppm. You would need to purchase Jack's Test Kit to monitor your concentration or you are "Walking in the Dark."
When I kept it at the minimum level, of 10 ppm stains would slowly reappear after a few weeks, at 20 ppm after a few months, but at 40 ppm it is nearly stain free after 5 months. We have a lot of brown volcanic iron blown dust into the pool, at certain times of the year our winds can blow at 35 to 50 mph for a week or so. When this happens I test the sequestrant twice a week, or more.
When doing an ascorbic acid clean in my 10,000 gal pool I find it eats up about 8 - 10 ppm of sequestrant.
If you do not have an automatic ph/Acid metering system I have found that keeping the pH at 7.3 allows for a margin of error.
What kit are you testing your balance with? This site has an excellent test kit by all accounts, as does Taylor Industries. If you are using another your figures could be off, if your chemicals are out-of-date the measurement could be wrong, by calling Taylor Industries with the code on the bottle they will let you know if they are still usable. Generally speaking I would not trust a pool store's results, especially if they use and electronic readout machine as these are quite difficult to calibrate correctly. My own experience with those machines has been far off the true numbers.
In addition, and I do not know why, that by keeping the CyA between 70 and 80 as recommended by this site, and the Chlorine at 4-6, this to helps keep the stains away. Again I do not know why but it does.
My personal numbers look like this:
Cl 5 -6
pH 7.3 -7.4
TA 80 - 90 (does not budge)
CH 375
CyA 70-80
Salt 3300
Borates 0 (dog)
Temp 72F
Phosphates are well over 20,000 ppb but they are no trouble so long the Chlorine is kept at 5 or above (max 9) or you will have problems getting a pH reading.
Hope this helps and Good Luck.