I think the size of the Luna 300 may be a contributing factor as to why you're having difficulty. My buddy had a similar problem with a Shimano Calcutta 400B while trying to cast light presentations, while Stratocaster and I were having no issues at all with the smaller Calcutta 200B. I've also got a Luna 253 which I use for coho, and it can also handle light presentations quite well, although I really like my Calcutta 200B for light presentations.
I guess my point is, larger reels need more weight to cast properly. I know you mentioned that you previously used an Abu Garcia 6500 without issue. My buddy had the same experience as well until he switched to the Calcutta 400B, then he was having all sorts of birds nests and problems casting. The Luna (and Calcutta) is a much higher quality reel with higher precision and tighter tolerances than your Abu Garcia. My guess is that you will be able to manage once you figure out the "feel" of the reel and develop muscle memory, but the larger sized reel just adds a level of difficulty for casting light tackle. Good luck!