That diagram seems to be confusing the 24 RMA retrofit (V)4 version with the full 37 RMA (V)1 version based on my earlier post:
Having looked at the numbers, I think the dB measurements are actually factoring in the gain as well as the power, which is dependent on aperture area. Overall received power is proportional to the Powerx(Gain^2) product and Gain is proportional to area. So the log of the RMA ratio cubed x10 should give the dB increase.
www.radartutorial.eu
So with 24 RMA giving +15db and 37 giving +20db
10log[(37/24)^3] = 5.6.
37 RMA is +5.6dB more than 24 RMA or +20.6dB
69 RMA should theoretically be:
10log[(69/24)^3] = +13.8dB
+15dB +13.8dB = +28.8dB
Probably enough to boil the brains of enemy pilots buzzing your ship.