I define the efficiency as million eu produced per fuel rod. This works well since you then can compare the different types of reactor designs.
If the reactor produces 120 million eu over one cycle using 20 fuel rods that would be an efficiency of 6 million eu/fuel rod. This way you can easily compare the efficiency per fuel rod between regular, mox and fluid reactors.
There is also a nice trick for calculating the efficiency of fluid reactors using this method as well. with stirling 1 hu/s=1eu/t and with superheated steam it is 1 hu/s=1,5eu/t. So lets say a 5 efficiency reactor(by my definition) produces 200 eu/t and produces 600 heat. In fluid mode that would equal 600 eu/t with stirling and 900eu/t with superheated steam. that means with stirling the reactor would be 600/200= 3 times more effective with stirling and 900/200=4,5 times more efficient with superheated steam. If we multiply this by the non fluid mode efficiency of 5 we get an efficiency of 15-22,5 million eu per fuel rod. For reactors that are not in even 100 the maximum we can do is 1,5 times even hundreds of heat then we add the remaining as stirling power. so 620 hu/s would be 900+20 eu/tick maximum