For the depth you are fishing I would recommend that you try a fluorocarbon furled leader about 7 foot long and attach about 5 foot of fluorocarbon tippet. Fluorocarbon will sink faster than mono leaders and I feel would get your fly down to the depths you are fishing. I make my own furled leaders and I nymph/wet fly/streamer fish year around on the local rivers, lakes and ponds and never change from the set-up I just recommended. I use the above set-up everywhere I go fishing and have had good success with it. My flurorcarbon tippet is 6 pound test attached to a 7 foot furled leader made from Berkley Vanish fluorocarbon that is 4 pound test.

The above recommendation is only my opinion and suggestion and others may agree or disagree.