if you were nail knotting your flouro directly to your fly line then stop doing that and instead put a small chunk of mono nail knotted to the fly line (25/30lb is good) and leave this chunk about 6-8inches long and tie a double surgeons loop in it. this will act as a butt section and a loop to loop connector. then double surgeons your flouro and loop to loop connect the two, add your fly and Voila !!!
of course if your fly line has a welded loop already then there is no need for a mono butt tied in, however i have yet to see a sink tip that has welded loops on both ends.