Here's the chart I use:
Roll a stress die; if the result is 21+, child is gifted.
For each parent:
For aura of gestation (choose the one most frequently inhabited):
This 5x value reflects an increase in dominion power in our own campaign. Using the standard wussy dominion numbers, this should probably just be - aura, which won't matter for magi in all likelyhood, but don't try and use it to chart statistical probabilities of gifted children in Mythic Europe, as the numbers will come out too high thanks to our friend the "1".
This results in approximately 50% of children born to two magi parents in an average magical aura (3) being gifted, which was the goal.
If you wanted to increase complexity, you could allow a second stress roll to determine magnitude of gift, from minor to full, but I've always assumed that's a result of training, so haven't bothered with it.