I agree with phyringe and femurat.
The lock should be enough. If you go higher the weak-point would be the locker.
A thief won't pick, he will cut the shackle.
Make your padlock look old shabby and dirty. Put a few flacks of paint or sth like that. Just be careful not to obstruct the working of the lock with it. (Keyhole!

)
I would still put a bike lock on the bike inside the locker. Even if they open the locker they should be at least annoyed.

Abus makes not too bad bike locks too. Just avoid anything with a combination lock or a cable. The ones with jointed bars is pretty good I believe.
So I think the lock you choose is good. I would get the 20/70, but I'm a lock enthusiast. I use an Anchor las (a very good and correspondingly expensive padlock maker, a total overkill for a wooden locker) on my locker. Ain't no fucker gonna steal my jumper and notes the easy way...
In case you need any further advice your country could be useful to determine which lock makers are available to you.

Come to think about it since you bought the 28/70 already just stick with it. It should be enough. Even I wouldn't buy another one if I had a 28/70 already.