I'd go early May too
One of the best days we have out, was early May on Monymusk.
And as the guys say, you can hang around and there may be some evening sport, although it may be with streamers.
Thats really good fun too, and a nice way to spread the day out.
If you are out, then the main hatches will be from 1130 -1430 at that time of year, LDO's and march browns and perhaps a small sprinkling of Iron Blues.
The other trip i would reccomend travelling down for would be Kildrummy in mid-late June.
You'll get some good nymph fishing during the day and then a lovely dryfly session later on with Yellow mays, BWO's and sedges. Very nice piece of water to fish too, with some cracking fish
Sandy