This math however assumes a few things that are not realistic. First of all, it assumes chain casting your spells so that you never leave the 5 second rule and waste no time between casts. Secondly, it does not account for double procs where a cast you make within the proc of blue dragon re-procs blue dragon. Thirdly, it assumes only using 2.5s cast spells when priests (and druids) will almost always be using a mix of 2.5s, 1.5s, and instant (GCD 1.5s) cast spells, and druids will sometimes use longer cast time spells.
In order to account for this disparity between idealized assumptions and impossible to reliably quantify real world usage, we can fudge the numbers a bit. Instead of a final value of 1 being the changeover from SotS to DCBD, we will arbitrarily use 1.5 and hope that it sufficiently accounts for the difference. If anyone has a better idea, I would love to hear it.