Here's a good article, maybe help get your math straightened out:
http://www.supercoolprops.com/articles/meredith_effect.php
It is an interesting article and parallels the model I used to get my 30-40 pounds of Net Thrust.
What the article leaves conveniently untouched are the Major assumptions and calculations of the pressure drag of the impinging airflow on the radiator as well as the drag of internal flow losses.
The Model must have a.) Mass flow - Inlet, b.) Physical dimensions of inlet, c.) Drag due to internal flow between inlet and radiator, d.) Radiator (both heat transfer to the passing fluid and pressure drag of flow impinging on coils), e.) Drag due to higher energy flow in Plenum, f.) physical dimensions of exit shutter for the flight conditions modeled.
He treats the model as a venturi tube model - and then calculates the Net thrust based on the delta between input flow properties versus exit velocity. This is a.) a Thermodynamic System, and b.) it is 'attached' to the airframe and, as such, a major component of drag in low to medium speeds.
Another observation that should be noted is that he is basing all of his mass flow calcs on 430 mph at Sea Level.. good for Reno, but about 60mph above top speed at 72" MP for a P-51B/D.
This is why NAA engineers separates the Cooling Drag discussions between High Speed and Climb and why they state in the Performance Calculations Report NA-8449. pg 21 of 62.
Cooling Drag
"The basic drag includes the external drag of the radiator dust for the flush exit flap position, but does not include the internal losses or drag due to change in scoop exit flap position. These added drags are given as follows:
Calculations show that for most cases a small amount of thrust is derived, however, for this work any thrust from the cooling air is neglected. High speed Cooling Drag is then considered equal to zero".
[/NOTE: I commented that the best I could extract from My assumptions was 30-40 pounds of Net Thrust. Joesupercool derives 270 pounds. Ya think NAA engineers would ignore 270 pounds of 'found' thrust in a Performance Estimate, an amount far in excess of exhaust thrust?BTW - to get 430mph at SL in a P-51D would take more than 2400 Hp ~ 1775 pounds of Thrust plus another ~ 200 pounds of exhaust thrust.