IMO it depends on some drops of rain and a small rise in the water level.
The fish are sitting in the lower river at the moment but with a little rise in water they will rise upstream. In August they often do not stop in Grantown Association water but run straight thru it to Castle Grant.
Apologies I thought Grantown association was called upper/lower Castle Grant?
On the Spey system there was 196 salmon for last week is that a good number for the time of year on the Spey?
I fished it day and night for a week in June and only caught one sea trout, i would need the odds to be considerably better to consider it.
Interesting theory, except Castle Grant is downstream of Grantown Association!
Stop thinking about it and go. It only takes one fish, and even if you don't catch you will still have a good day, have you fished it before?