I have to disagree with Hickory. The best time to dualclass the Kensai>Mage is either at level 9 or 13. Level nine to get the most out of the HP boost, and an extra weapon profiency, or level 13 to get the extra half attack per round from being a warrior, as well as additional combat bonuses from the Kensai kit. Kensai13>mage 28 is considered the best by most powergamers.
Also dualclassing at level 8 won't get you any more spells than a kensai 13>mage. However you will be more fragile and less deadly in closecombat. It's your choice.
Leveling from mage 28-31 only Awards you with a measly 3 HP(no extra mage spells) compared to 5 extra levels of Kensai which gives: one extra weapon profiency, +2 to thac0 and damage, an extra half attack and 16 extra HP!
While the Kensai/Mage is the better class late SoA, a multiclass completely destroys any dualclass char in ToB, since a dualclass reach their peak just at the end SoA/beginning of ToB and really stop advancing, where as multiclasses gain huge boosts each level up.
Everybody has their own opinion, and I don't necessarily disagree with what you are saying, but my point was that at the levels I stated your mage would be able to progress to maximum level.
BG1 (TotSC) XP cap 161,000
Pure Fighter max level 8 == 125,000
Pure Mage max level 9 == 135,000
Fighter 4 (8,000) / Mage 9 (135,000) == 143,000
MAXIMUM MAGE LEVEL
BG2 & ToB XP cap 8,000,000
Pure Fighter max level 40 == 8,000,000
Pure Mage max level 31 == 7,875,000
Fighter 8 (125,000) / Mage 31 (7,875,000) == 8,000,000
MAXIMUM MAGE LEVEL I am not a power gamer, and therefore don't think like one.