Overfilled! Amt of fuel through return line?

[Rooster300SD]

I run WVO in my 84 300SD and recently set up a fuel cell in the trunk
to start & stop the car on thinner fuel. I put in a 5-9 psi fuel pump to
help bring the fuel up front. I noticed my 2nd tank going down rather
quickly and figured it was getting returned to the original tank.

Well, tonight I noticed a fuel leak under the car and realized
it was because the main tank had overfilled! I had recently
topped off the main tank with WVO.

My question is: how much of the fuel gets returned to the tank
during normal operation?

Thanks,
Jeff