Water Softener - How to calculate regeneration?

What is the formula for calculating water softener regeneration? If I know the grains per gallon (hardness) is 15 ppm, and the size of the resin bed is 41.6 pounds of resin OR .8 cubic feet AND the RESIN cylinder is 10" diameter by 35 inches tall. And if I can read my water meter to see how much water I am using daily.
How do I calculate how often the water softener should regenerate (e.g. in terms of water useage)?
Thanks in advance,
Andy
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Not really knowing what I'm talking about, but I think you also have to take into account the dissolved iron in the water. so much ppm of iron equates to so much grains of hardness.
Check here if you don't get any answers: http://www.qualitywaterassociates.com/phpBB2/viewforum.php?f=2

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