I'm taking the same approach (limited access, advanced skill). I have my players roll 2d10 and add them up. Subtract this amount from their POW. If the result is negative, no starting common magic. If positive, divide by 2 to determine the amount of 'Magnitude's' they start with to determine common magic. Then they roll on a Common Magic chart to determine the first spell, choose the second (if they have Magnitude left), roll for the third, choose the fourth, etc. For progressive spells players can decide what level of magnitude they want in it up to a max of POW/6 (i.e. magnitude 1,2 or 3).
For example, Alaric has POW 16 and rolls 2d10 getting 5 and 4 for a total of 9. POW of 16 - roll of 9 nets 7, which is divided by 2 resulting in 4 magnitudes. He rolls on the chart and gets Abacus (magnitude 1). That leaves him 3 magnitudes left. He then chooses his second spell, taking Bandit's Cloak (progressive). He can take it at up to magnitude level 3 (POW 16 / 6 = 2.67 rounded to 3). He takes it at magnitude 2 leaving him 1 magnitude left. He rolls again and gets Befuddle (magnitude 2) - as this is more than the number of magnitudes left he cannot take it. He then chooses Countermagic as his last spell and gets it at 1 as that's all the magnitudes he has left.
In determining spells I always alternate between roll and choice so that if, as in the example above, the roll results in a spell which has a higher magnitude than what's left, the player can't take it but his/her next spell is a choice.
This approach rewards higher POW, provides some flexibility for players but also limits the number of initialc ommon magic spells available. The max you could get, assuming an 18 POW and a snake eyes roll of 1+1=2 would be 8. (POW 18 - Roll 2 = 16 / 2 = 8).
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