We don't know anything about using "old Tango for DOS".
First, consider the fabricator of your part as the THE most appropriate place to look for layout recommendations. We've discussed the heirarchy of sources for layout information previously here on SMTnet.
After that, by "define clearance around the surface pads" do you mean solder mask? If so, NSMD. Make the solder resist aperture layer between 0.004- to 0.006-inch larger than the BGA pad to give normal clearance. SMD. Typical overlap would be 0.002 to 0.005 inches. [Your fabricator should have input regarding their process capabilities.] The reliability of this type of solder joint is still in review.