The Canada Safety Council course teaches us that 70% of the braking power should come from the front brakes and 30% should come from the rear brake. However, this % can vary depending on the traction available and the type of bike.
On a road race track 99.9% of the braking comes from the front brake. The weight shift to the front wheel under braking will cause almost 100% of the weight on the front, leaving the back wheel barely in contact with the pavement (see picture above). With no weight on the back, the rear brake is pretty much useless.
A sportbike is relatively short so it reacts to weight bias changes pretty dramatically. An adventure bike or dualsport bike with relatively soft, long travel suspension will also transfer weight pretty quickly as the front end dives under braking. These bikes will favour the front brake when the traction is good (90+%).
As the wheel base of the bike increases there may be less emphasis on the front as the effect of the weight transfer from back to front decreases or slows down. This is when you will return to the 70/30 ratio. As the available traction decreases the ability to use the power of the front brake will decrease and you will have to rely more on the back brake.
Once again practise is the key. Find a parking lot and try stopping with just the back brake. Then just the front. Finally try using both brakes. Make the proper procedure your reaction and your muscle memory.
