What I always find surprising is that some oncos give salvage RT without HT. In your case, this makes it quite hard to know what to expect in terms of PSA results as all the common data is about PSA after RT / HT combo.
What is definite is that a) PSA doesn't always drop immediately after RT and b) RT continues to work for up to 18 months after the treatment so you expect to get your nadir a year after treatment or even later (in John's case, it was 2 years post salvage RT)
As Chris says, you will have a better idea in December.