Reptocal contains phosphates which is already available in just about everything the turtle eats. Reptiles need a 2 to 1 ratio of calcium to phosphate. Excess phosphates will inhibit the utilization of calcium by the turtle. I would recommend Rep-Cal instead since it does not contain any phosphates.
Reptocal contains phosphorus, a product like RepCal does not. I'd use a calcium supplement that doesn't contain phosphorus rather than one that does. RepCal is good, there are other calcium supplements that don't contain phosphorus as well.
"You become responsible, forever, for what you have tamed." -Antoine de Saint Exupery-