Years ago, when I was fairly green at this game, I had recently purchased a 13' 6/7/8 CND Black Spey. I started out with an Ace/Vision 450 gr Scandi head coupled to Rio
15ft 95 gr type III & VI sink tips. I had previously used A Rio
450gr "beer can" Skagit line with those tips on a 12ft 7/8 rod with a modicum of success. I was having a hard time with the Black Spey rod, but since I had paid a lot of money for it, I was determined to unlock the code & make it work. Part of the frustration was due to the fact that not only had I learned to cast long belly lines, I was also wanting to move to intruder type flies.
People on the forum were starting to talk about Al Buhr's book on cutting & splicing fly lines, so I bought the book & began to accumulate bargain lines to cut & splice. Al's book has all the information, the why's, the formulas, everything, to build a line that will work, but it takes a while to soak it all in. After building several lines that worked fairly well, just about the time I was starting to get it all together, Tim Rajeff developed the Airflo Skagit Compact series heads.
When I had the opportunity to try several Skagit Compact heads of various grain weights, coupled to different sink tips of varying grain weights, always tossing the same "truder" fly, things started coming together on that rod. What seemed to work best for me was the 540 gr head coupled to the Rio
(10 wt) 150 gr type 8 sink tip.
Having also tried the 510, 570, even the 600gr heads, coupled to various lengths of T-14, I have settled on the 540gr head & T-14 for casting weighted "truders" on that rod. As the water conditions or the fly changes, I can forgo the T-14 for regular "type" tips, and/or decrease the grains and go to more of a Scandi type head. That Black Spey has become my go to rod.
The sustained anchor cast is a completely different technique than the long line cast. The Skagit cast is not quite the same as the Scandi cast. Concentrate on one type cast & a setup maximizing that style.