Yup. Figure out how much data you would write to T1 in the span of time between the snap schedule you want and then add whatever for growth and you have  your T1 capacity requirement for a multi flash tier config anyways.
Random Question, When Tier 1 fills up, does Compellent write to Tier 2 directly? or will it first move data out of Tier 1 and into Tier 2 and then commit the write to Tier 1?  Cheers
Tiering Question