My current set up is @ 40' of DT 11 and @30' of DT 10. Loop your sink tips directly to the front of the DT 10, or add the front taper from a Mastery Spey for greased line work. Match the font taper of the DT 10 to the back end of hte Mastery Spey front taper for best results.
I like Bruce's formula (just tried it last weekend at the M&Y Spey school--works very well on the new Scott 15' 10 weight we were casting!); I might be inclined to add my DT line between the front taper and main belly of theMastery Spey, and to have that new section of line drop down a line size from the main belly--if the belly of the Mastery Spey 10/11 is #12, for example, add # 11. Again make sure that the tapers are matched (use a thickness guage or a micrometer for this).