That is not the rule of thumb. The rule of thumb is to divide beta by 10, not the collector current.
I like the theory, but don't think it's right. Using it would give his transistor a gain of 1.5 at 10 amps. I know it is not the hottest transistor in town but it is not that bad. The data sheet states a gain of 5 minimum at 10 amps.
If you look at the 2N2222 which has a little better spec sheet the vbe Sat is speced at Ic=10XIb. And it only has an Hfe of 50 at 10ma.
But to Amir1's concern: You choose the base current for the maximum collector current tou expect to see. In your case 8 amps. If it is saturated at 8 amps it will be saturated at 2 amps.
Mike is right, this transistor is not the best for your use. It will waste a lot of power. You could use a P channel fet for the high side and protect the gate voltage from being to high with a zener. This would also simplify your driver. Last, don't forget clamp diodes for the motors inductive kick.