Yes, the amount of starter culture is all over the place. The manufacturer says that the 25g packet is good for 200 kg (440 lbs) of meat.
This translates to about 18 lbs/ 1g. From this you can see that for 5 lbs you would use 0.27g 🙂 Which is as you may realize a ridiculously small amount to measure with any teaspoon.
You can not go wrong with using a little more culture to make sure it incorporates evenly into the meat. As for Ruhlmans Book asking for 10g (I should let them address the reason why they recommend using this much).
For us 1/2 tsp works well for 5 lbs. of meat.