First off, welcome the rangers club!!!!! and second, that longbow is very nice,
search the ranger builds listed here and will see alot of great ideas, your bow
does not have to be perfectly setup to use for different things, your player
skill and skill selection are more important.
Build selection depends on certain things, are you soloing, or in a group
on missions?? what kind of monsters are you facing, i.e. poison does not
work on undead, but fire is great. As far as marks req. goes, I believe you
need to meet the req. to get max damage, not sure what all it affects.
Oh, and shadow bows have a built-in 10% armor penetration all the time,
along withe horn and ivory bows.