First off I was wondering how much 50lb mono I can spool on my senator 4/0? I know it holds like 475 of 30lb mono but I don't know the equation to get the amount of 50lb it will hold. Also I'm wondering if an 80lb test leader with a swivel and fish finder rig will work well for targeting the bigger snapper, grouper etc...? By the way I am planning on fishing channel two so any advice with what I should do will be greatly appreciated, thanks!

80 lb leader is too heavy for snapper on the bridges IMO, those fish see a lot of pressure and are leader shy...if I were you I would fill up that 4/0 with 40 lb mono and use a bottom rig with 50 lb leader. You will have a better shot at the snapper with that. Keeper grouper are not very common down there from the bridges, I have caught 2 in about 150 trips.

I've caught zero keeper grouper in my 500 billion trip.

Okay got it I will use 40lb mono then and a 50lb leader...I had some 80 that the guy I bought my senator off of gave me but I'll save that for something bigger I guess. Also do you know how much 40lb mono it will hold? I just don't want to buy too small or slightly big a spool and waste line or not get enough line. thanks!

Will not be much less than the 475 with 30lb.

Not sure but maybe 420 to 430

First you have to check with Penn & what line diameter at 30lb where they using since each
monofilament manufacturer line diameter is different.

Lets say they were using 0.22 line diameter of 30 lb for 475yd.

Andes Pink line:

If they were using 30lb .022 & getting 475yd
Then Andes Pink @ 40 with a line diameter of .024 gives you 399 yd

http://www.accuratefishing.com/support/linecal.php