Good formula.
Let's extend our analysis a bit.
Let "x" be transaction rate, i.e. number of transactions per second. And "t0" - average transaction validation time, i.e. block validation time divided by number of transactions in that block, in the first approximation we can assume that "t0" doesn't depend on block size.

Than, number of transactions to be validated N = x*T
and time required for validation t = x*T*t0

Thus, orphans rate, expressed with theese variables: 1 - e^(-x*t0). We see, in first approximation it doesn't depend on T. Dependacy on T arises from network delays.

I agree, that working on payment channels is important. But may be it would be beneficial for Bitcoin to adopt shorter block times. At least it definitely would eliminate that marketing issue.