If USSWIT10 Curncy is the expected average inflation for ten years and the USSWIT5 Curncy is the expected average inflation for five years, I don't know why that formula would tell me what the expected 5 year inflation rate would be 5 years from now. Shouldn't there be some sort of compounding taken into account?
I almost feel like you should have to take ((1+USSWIT10)^10 - (1+USSWIT5)^5)/5. Something like that.