2.5wpg should be enough, i found that a good substrate seems more important. I have Eleocharis parrulus and Eleocharis acicularis in both tanks, One with a sand substrate (120L) and the other with a Sera floredepot and jbl florapol mix and black fine gravel (60L).
The groth rate between the two is unbeleavable. In the 120L the grass looks healthy but tends not to carpet, in the 60L its carpeted the whole tank.
The bigest problem i have is an out break of hair algae, once tangled in the grass it a pain in the a*!$ to get out.