Except for slow USB stick miners you do not need to set difficulty. The pool takes care of that automatically.
Though, I would suggest he set them anyway, since they are high hash rate miners.I set all my miners to 42000 so they don't get a rush of low diff shares when they first connect.
Yes the pool will switch it up almost immediately, but, alas it tells the miners to use the default 8190 before it switches them up.
You'll see the value they settle at on the Workers->Workers page, which will be a lot higher than 8190,
so setting their default higher helps with the initial connect.
Checking mahdi13709's page - they are all mining above 60000, so a start of 40000-50000 would work well.
For PPLNS, it makes no difference to your rewards, but does mean the initial connect will be smoother;
and on Solo the one share you are looking for will be higher anyway
Again this only makes sense if they are high hash rate miners.
