The process for extracting castor goes something like this. The beans are first dried then go through a press to squeeze out the majority of the oil (something like 80%?). The crushed beans are then collected and re-pressed which extracts maybe another 10%. That gives the first and second pressings. That oil all goes into settling tanks and filtration which is also part of the de-gumming procedure so you end up with second pressing de-gummed oil before it even leaves the manufacturing plant. The final extraction process can't be done by pressing so chemical extraction is used on what remains in the bean husks. This chemical extraction leaves tiny traces in the oil which apparently is what causes the white flakes in fuel in very cold weather when this oil is mixed with second pressing oil.

As far as mixing fuel by weight, I'd heard that Byron used this method and some have done evaporation tests on Byron fuels which seems to confirm this. Mixing by weight is actually a more accurate way to do it but only if the end result is to give a fuel that's the equivalent to the normal method of volume at a standard temperature. Mixing by weight and using those weights as a percentage with no explanation is both misleading and almost criminal because volume is the standard that's used and understood by all modellers.