Barding difficulty of a tamed cu sidhe usually ranges 95-100. Discording it takes about 5 points of difficulty away with 120 discord skill... the less discord skill you have the less reduction. For example, a 125.9 Barding Difficulty (BD) untamed cu sidhe went to 96.9 tamed BD, then to 91.8 BD tamed and discorded. (I just happen to have an Excel spreadsheet with a bunch of cu sidhe date on it... heh).
The skill needed discord the cu sidhe depends on the starting BD of the cu sidhe, the instrument, music and discord skills. The highest untamed cu sidhe I've seen was just over 131 BD to about 100.8 BD after taming, and the lowest was about 120 untamed to about 93 DF tamed.
The Barding calculator doesn't take the varying DF of the cu sidhe into account (just the average I believe) but shows that with 100 music and 94.5 discord skills, you have about 5% chance to discord. 100/100 skill gives you about 16% chance with a normal instrument. Exceptional instruments add 10% success, of course.
Is that what you are asking?