Consider an example:
mn:/15+4 should fire every 15minutes at 04,19,34,49 (but actually goes off at 11,26,41,56);
mn:/15+11 actually does it right.
The text in help is right:
"A spec can be a number, * (will match every), or in the form of /n+m, which matches when (value+m)%n = 0"
But the example is wrong.
In the example “mn:/15+4†, the match occurs when (clock minutes + 4) is a multiple of 15.
The device doesn't seem to recognise "-" in the Config String.
A better formula for event at clock time "n (multiple) + m", would be "/n+x", where x = n – m
So “mn:/15+11†achieves the desired result.
I’ve also found that when you start to enter text in the Config String box, the first character isn’t visible (but is actually there).