An easy way to calculate the amount of energy available at each level is to

Use the rule that 10% of the energy is transferred between layers.

Explanation:

Energy is transferred between layers of a food pyramid. That means that the  producers at the bottom of the pyramid (e.g. green plants) provide energy to the primary consumers (e.g. rabbits), which are eaten by and provide energy to the secondary consumers (e.g. foxes).

However, very little of the energy is actually transferred to the next layer, roughly 10%. So an easy way to calculate the energy available at each level is to calculate 10% of what was available from the previous level. So if there is 600 kJ available from the primary consumers, then 60 kJ are transferred to the secondary consumers