The pool will automatically adjust the difficulty
unless you connect with Usename.workername_diff256 (256 is an example)

so you can tune the difficulty manualy.
For 15Mh/s miner, it depend on what device it is.
if it's a single device capable of 15Mh/s  (higher diff needed here)
or if it's 3 gridseed blades(3x5Mh/s),  (lower diff needed here)
you will not set the same difficulty.

it also depend on the block time of the coin.

if you put 128 diff for WDC (30sec), you will put 256 for FTC (60sec).


For WDC and gridseed blade, I think the best result will be 256.

The best way to set the difficulty manualy is try different values and choose the best.