Static Difficulty

Hi I recently came online with a few D3s. I was considering SOLO MINING on PH to compare profits. But i do not know where to find the appropriate number to set my static difficulty to.

I see that on the automatic pool mining mode my D3 difficulty shows 32(dynamic) and most of the time its at 32. but in the help center it says D is between 512 and 2^21

How do i find this number for static difficulty and how often does it change?

Thanks,