I have an Abu 6501 C3 (lefty) which I will be using for trolling eels, rubber, and tube&worm from the kayak. My question is how high should I load the spool with mono? I never got the hang of casting this reel, so it is now the designated kayak trolling reel.

I never had a problem casting any of my 6500 or other ABU models. The Ultracast spools are fast, make sure that you have the right spool brake and adjust the spool tension knob. If that still doesn’t help you can try using heavier oil in the spool bearings or even attach a small thin magnet(s) between the spool and the side plate.

When I spool my reels I generally fill between 1/16" and 1/8" from the spool edge, You can add less line but then you looses retrieve speed.