I don't see the point..I drew this out because it had the LERX shown in 2011 and later models, whereas other diagrams tend to have round LERX as on the 2001.Honestly ... You "need some measurements to get this started" but if you want to make it correctly by comparing length of and ratio between esp in order to compare and the deduct whatever you want, I would never start with such a crappy drawing, which is all wrong!??
But even if you go by photographs, you still end up with an inlet only length in the 7-8 meter range.
I think the overall point of this thread is to explore what the inlet means and how it applies to the J-20's operating characteristics.
Check out this NASA page:
Here, the difference between a supersonic and a subsonic inlet is defined. In a supersonic inlet, you want more length so you have more space to bleed out the boundary layer and dissipate shockwaves. For subsonics, on the other hand, you want as short an inlet as is practical for the reasons given above.
The long J-20 inlet is suggesting one or both of two things: first, the J-20 can supercruise if its inlet can keep the AL-31 / WS-10 working at high efficiency. You only need about 50 kN thrust per engine to break the Mach barrier,at altitude, and the Al-31 / WS-10 should have around 75kN base thrust. But that's a sea-level rating, not a rating at altitude,
Second, the long inlet design is an optimization for high-speed fighting, not low-speed fighting. The longer your intake is, the more your engines are going to have to suck, and until your suck has been overwhelmed by speed-dependent blow, you're at a disadvantage.
You know what other aircraft has long intakes?
The point being, regarding the MiG-31, is that its inlets are designed to maintain usable airflow to its engines at high speed, i.e, it draws in a lot of air.to work as altitude, and is long enough to absorb multiple supersonic shock waves. The J-20's inlets are somewhat smaller, but it has a similar length to that of the MiG-31, with the latter having around 14 meters of length