Most of the time at 50 down we run out 20ft, as far as flashers go run the flasher off of your flouro lead, and then a lead from the flasher to fly or spoon that is about 2x the flasher length, Also imo you can save yourself alot of headache by just running 15# to 20# power pro with no flouro lead, we have experimented with the flour leads, mono leads and no leads over the last three yrs rigging and have found no difference in catch rates from one to the other, unless you are running real shallow.
hope this helps