One suggestion I'd make is trying a mini band stretched around the back while db benching. I haven't actually done this yet, but I plan on picking one up from elitefts.com soon as I use only dbs for benching (haven't bb benched in a few years).
I think the advantage of the mini band is that it will allow you to address one of the disadvantages of the dbs in that you start in the stretched position--the band will make the resistance lower at the bottom and higher at the top, thus minimizing that issue. Also, it will allow you to use a lower weight in the db, which can be an issue if you're working out somewhere that isn't "hardcore." Another advantage is that it will give you a variation on the movement, and that is always a good thing... now if I can just figure out how to attach chains to dbs.
I wouldn't use the band all the time, but it would be good to mix it up, and for ten bucks you can't go wrong.
On the db to bb ratio, I think it's a personal thing, but once you figure out what your ratio is, it will be consistent for yourself. For me, 110x5 with dbs was ~305x1 with the bb (haven't bb benched in awhile so I don't know what it is now).