What Sort of Calgon?
Are we speaking of? New or old product? Liquid or powder?
The new powder is mostly sodium citrate and sodium sequestrate (sp?), IIRC or maybe sodium carbonate as Calgon replaced phosphates on all it's powders ages ago. The liquid is mainly sodium citrate.
Anywho, start with 1/2 tablespoon and either increase or decrease as results warrant. Remember to use slighly less detergent to compensate for the water softening of added Calgon, or you may end up with poor rinsing results caused by too soft water.
L.