This is kind of generic for all the masteries though the weights may change.
For the primary skills (Music + Mastery Focus Skill)
You get 1 point per 10 skill points above 90. (and since the minimum requirements is 90, you automatically are starting at 2 points - one from each skill)
So from musicianship and your mastery skill you have up to 8 points.
Your secondary skills (other bard skills than your mastery skill) you get 1 point per 10 points above GM, so thats a potential of 6 points.
The weighting of those points vary by spellsong and may be a factor or exponential.
For example
Inspire would be weighted at 2 per point of primary skills and 1 per point of secondary skills.
So thats (8 * 2) + (6 * 1) for a 4x Bard to HCI/SDI -> 22%