Depends on your flippers. Newer games have stronger flippers so 6 degrees or more is better.

You can always try it, and if you aren't happy with the shots, lower it back.

LTG : )

I believe it is correct what I've heard- that the factory angle is when the bottom of the cabinet is level. Or when all 4 levelers are equal on a level surface. Any setting different than that is arbitrary. Me? I like a bit faster play so I jack the back by about 1/2 inch.

If you haven't already, apply a generous coat of wax. A good coat of wax can make a huge difference on a game that old.

My Atlantis is at 4% only because there are cupped inserts and not getting them hung on them. The only thing I am missing on this machine is the evil dump (no return wire) when you try to trap on the right flipper.
My Abra-Ca-Dabra is at a 3.7% slope and I may lower it to 3.5%. I need to have it a few more weeks before I decide what slope I like best. Others will chime in. If others have a slope they like on their Abra then I would be most greatful to try them out. I just did a full shop and putting the posts and rubbers back on as I type this.

What ever angle you can handle!

I don't really pay attention to the angle. I just keep tipping it up until it feels right.

I haven't measured the angle on mine, but I know it's pretty steep. Maybe 5%. I use 2" levelers on the front legs turned all the way in with slight adjustment for side to side leveling. The back legs have 3" levelers turned about 1" in, again with side to side leveling. This is with Varathane and all inserts leveled (and flippers rebuilt+high tap). needless to say, there can be short ball times, but plays pretty fast at times. You really need your coffee before you get behind the wheel!