This test is completely unnecessary. You are making fuel, not a pure laboratory product. Using biodiesel, instead of WVO, is all about viscosity. The efficiency of conversion is therefore immaterial - as long as you have sufficient conversion to reduce the viscosity enough so that your car will start on a freezing cold winter morning, that's the only test you need. Personally a far more useful test would be to put the fuel in the fridge after you make it. If it doesn't gel (which it may if you don't have full conversion) you're good to go. That's not to say that there is not some clever chemistry behind this test, because there is - you just don't need to do it.