a torque wrench that is normally used to do up to 150 ft lbs is going to be horribly inaccurate at 8 ft lbs. Each 1 lb error is going to be 60 ft lbs error after your multiplying by 60 times. I would expect that your wrench, set for 8 ft lbs, will return somewhere between 5 and 10 and you'll never know which... why not just use it to undo things, the way it's supposed to be used, and do the nuts up the old fashioned way? 4 foot breaker bar and lean on it?
This plus the fact that torque multipliers also have frictional loses (usually specified by the manufacturer) so you can add that to the mix and X it to your torque wrench's inaccuracies.