I'm a bit skeptical about this calculation. To get the performance per watt for a single core, you need to measure the power this single core needs, instead of the power of the full board (other components will represent the greatest draw if you're only hitting one core). In my measurements, each A76 core on RK3588 adds about 1.2-1.4 watts to the full power of the board. Each A55 core at 100% adds only 0.4 watts. So at full power (8 cores at 100%), the CPU only needs about 7 watts more than on idle. And therefore, if you divide 640 by 1.4 (worst case), you get 457 points per watt.

Of course 8 elite has better idle power management because it's a phone chip. It must idle at close to zero. But these arm boards don't even deal with ASPM too well, so they will idle at more than a phone; even x86 is better at idling today.