When I've covered bras with fabric, I've never used a pattern. All bras are different so I think that a standard pattern wouldn't work with all of them.
I just kind of eyeball the bra cups and then cut two triangles that are bigger than the cups themselves. I make the bottom edge of the triangle round instead of straight because I can get it to fit the cup better this way. And then I pin the triangles on the cups hand sew them in place making little folds or gathers where necessary but making them as invisible as possible. Then I cut off the excess. I do the same with the rest of the bra.
Here's a good website with another great method for covering a bra:
http://www.nadyas-sewing-instructions.de/indexe.htm
Click on the left side menu where it says "Tops&Bras
Regards
Priscilla