This might be helpful as sort of a baseline. This is straight from Rio (Farbank) website describing the Skagit MOW tips. First the descriptions:
The Extra Heavy MOW Tips are the heaviest of the MOW series; built with T-17 for the sinking section and with a sink rate of nearly 10" per second, it really gets down when depth is needed. Each of the tips either have a gray floating section, or if full sinking are a dark gray color, for easy identification. These tips are ideal on the heavier Skagit lines of
675 grains and more.
The Heavy MOW Tips feature T-14, with a sink rate of 9" per second, for the sinking material. Each of these tips has either a light blue floating section, or if full sinking are a dark blue color, for easy identification. The Heavy MOW Tips are designed for the large flies and are ideal for Skagit lines of
575 grains and more. There are six unique tips that make up the series.
The Medium MOW Tips feature T-11, with a sink rate of 8" per second, for the sinking material. Each of these tips has either a light green floating section, or if full sinking are a dark green color, for easy identification. The Medium MOW Tips are designed for heavily weighted flies and are ideal for Skagit lines between
475 and 575 grains. There are six unique tips that make up the series.
The Light MOW Tips feature T-8, with a sink rate of 7" per second, for the sinking material. Each of these tips has either a white floating section, or if full sinking are a dark red color, for easy identification. The Light MOW Tips are designed for lightly weighted flies and are ideal for Skagit lines of
475 grains and less. There are six unique tips that make up the series.
And here is their chart. Unfortunately, they don't add the advised grain weight, like in the description, to the chart. Also don't list the actual grain weight of the tip itself. Seems silly they don't, but maybe they thought it would be less confusing that way. Who knows.
I just know that if your spey or switch rod likes a 475gr weight Skagit line you should use a T-11 or T-8 head for best casting results. And if the 12.5' T-11 is not getting deep enough then try a heavier fly or feed line or something other than just jumping right to a 12.5' T-14 and having a terrible time casting it. Or worse, broken rods from hitting it with a heavy fly. On the beach we call that getting "Clousered". Not good.
Same with a 575gr rod. Use T-11 or T-14. Don't use T-17 and expect great results. It might work for the fishing you want to do, but it's gotta work for casting first.
