I have asked CASA about this since the type-in answers were introduced. Their answer was not much help. They simply said that they have various margins for full marks that they apply to these question. It depends on the question and the type of calculation and number of steps involved to get the answer. That's not much to go on.

All I can say is keep the decimals as they occur and round out the final answer. It's hard to see how calculating an answer to three decimal places off a kilogram is any practical use to a pilot. The main problem area seems to be extracting data from P charts. Just be very careful with interpolations and use a sharp pencil.