In the True Lineages official book, the Mercere Magi who are members of the Mercurial cult may learn the "Mercurial gift" spell. That spell is a ritual which give one character (for the current year) more chances to have Gifted children.
Do you have any idea how many chances that spell adds ? If a casual character basically get 1% chances to have a Gifted child, does the targeted character pass to 2% ?
The spell costs a fair amount of vis pawns (5) and I'm not sure how to balance it. By the way my players tends to get around 5-15 vis pawns per year.
The chance of any given child being Gifted is never stated in the rules (I've checked Apprentices, which confirms this).
Therefore you need to make up the number for randomly being Gifted, and then increase it. The number it gets increased to can't be very big, because there's only a dozen Gifted Mercere in the order and they would have far more members if this spell had a large chance of success.
In a previous topic we concluded that there is only one gifted human for 10000 European inhabitants.
Which mean each both the father and the mother have to get a natural 100 with a D100 roll (1% for each relative).
Some said a Gifted Maga can increase it to 10% chance (basically a ten on a die roll) which means two magi can only have 1% of their children as Gifted heirs. It seems fair to increase the difficulty to a 9+ with that ritual spell.
This came up in my home saga a few times. We decided that for important characters (The magi and companions) the player can just kind of decide for the story. For non-important characters (grogs, crafters, covengoats) they are not going to be gifted unless a player wants to make-and-play one as gifted. The player involved can also just decide to roll because the random chance is fun, and for those we settled on 1% as a good baseline to base on.