Just get yourself some hemostats if you don't have them already. Clip them onto your fly and drop them to the bottom to measure the depth. Grab the line where it meets the surface and peg your slip bobber down 12"-18" (this gets the fly in the zone). Bring the hemostats in and then start fishing.
I have a fishfinder and I still use this method...way more accurate and 6" can be the difference between a slow day and a 30+ fish day. This time of year the fish will be more distributed throughout the water column, but when the water cools off (Oct-Apr) 90% of the action will be within 18" of the bottom. I recommend just getting the 2 pack on amazon...cheaper than any sporting goods store, they get the job done, and you will inevitably lose a set to the bottom of the lake. They are also the best tool for removing deep hooks.
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