I've fished it a few times and Mike is on the nose... push off, cast out yer flies and short line your way around the loch.
It's all good, my best is around 2lbs but expect more in the 8oz to 14oz bracket.
Claret Bumble and a very bright soldier palmer did the best for me by a country mile
As I always say... don't be afraid to 'give 'em a mouthful' I'm seldom fishing smaller than a 12. If the weather is fair it should be OK, but I haven't fished Ruthven earlier than June and it can get chilly up there.
H