If I have a device drawing 300mA @5V and I need to use a 15V PSU what current rating do I need for the PSU?
device drawing 500mA @12v
The device drawing 500mA @ 12V uses a linear regulator. For a linear current regulator applies: the current drawn at the output is also drawn at the input of the regulator. So, it will draw 500 mA (plus a small additional current for the regulator itself) from the 15V PSU.
Note the difference in input power and output power is dissipated in the regulator: (15V-12V) * 500 mA = 1.5W. This requires using a heatsink for the MC7812.

device drawing 300mA @5v
The device drawing 300 mA @ 5V draws its current directly from the 6V supply through the
is current limiting resistor. So, again, output current is input current, and 0.3W is dissipated in the resistor.
So, current drawn from the 6V supply is 300 mA.

device drawing 100mA @6v
The 6V is generated by a step down converter 62180 fed from the 15V PSU.
Note to take in account the 300 mA from the 5V as well. So, the 6V supply needs to provide 400 mA.
From the datasheet of the 62180 you can estimate from Figure 11 that the efficiency is about 90%.
So, 6V * 400 mA = 15V * I * 90%. It will draw 178 mA from the 15V PSU.

In total, 500 mA + 178 mA + trickle charge current is drawn from the 15 V PSU, so, somewhat more than 10W.