RH for Simonis

I have a Diamond Smart table in my basement covered with Simonis 860.
I live in SE PA. In the winter the temperature in the basement is between 67 & 68 degrees and I boost it up to 69/70 when I go down to play. The basement tends to be damp but I have a good dehumidifier. I am maintaining the RH between 32 & 40 % which seems to be fine for the playability of the cloth, but expensive. Am I over doing it? What is the optimum RH for 860 at this temperature range?
Thanks

Rick,
Do you mean that my cloth will not lose any speed at 50-70% RH with the room temp at 68-70?
Thanks,
Jim

Thanks, I'll give that a try.
Jim