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It's not necessarily a requirement to use two flow meters to get a suitable flow reading on diesels. Years ago I used a Zemco car computer which was designed to provide mpg readings by calculating fuel flow using a single sensor comprised of a plastic ball in a race and an optical sensor. I installed this on a vehicle which I built that had a Perkins diesel in a Chevy Vega. My solution to dealing with the return line issue was to use a float bowl from a Holley carb. The return line dumped into the float bowl and the IP drew its fuel from the float bowl as well. Thus I was able to put the fuel sensor on the make up line to the float bowl and only read fuel used at the sensor. This actually worked very well and gave accurate readings.
There are two issues with going this route. The first is that many, maybe all diesels use fuel for cooling the injectors. It's possible that the fuel in the float bowl would become overheated after awhile and cause problems. The other issue is that many diesels use a lift pump which draws fuel from the tank under vacuum. It might be necessary to use a small "clacker" low pressure pump to feed the float bowl.
Another potential solution would be to use a surge tank of sufficient size and construction to provide cooling of the return fuel. By dumping return fuel into the surge tank the lift pump would automatically replenish the tank as needed. There could be a problem in using a lift pump for this setup related to getting air into the system that would collect in the surge tank creating a problem. It might be necessary to use a small boost pump to deliver fuel to the engine under pressure rather than under vacuum.
Creating a system that doesn't need to do differential calculations of two low flow rates is significantly more accurate than differential systems.
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