Your understanding is completely incorrect. Volume is calculated by multiplying the transaction price of the futures index by the accumulated amount of 300 yuan. The calculation of the transaction amount does not need to be multiplied by the margin ratio. Most importantly, your profit and loss calculation is based on the total contract value, and the margin ratio only allows you to reduce the amount of funds invested in trading, thus achieving a leverage effect. Therefore, the transaction volume is calculated according to the total contract value, which can also be understood as the number of transactions on that day multiplied by the average transaction price of futures index and then multiplied by 300 yuan.

It must be noted that the total position of the futures index is not necessarily related to the trading volume. Most importantly, futures trading is a T+0 trading rule, and positions can be closed on the same day. Positions only represent the number of open contracts. If you want to study how much funds are involved in the futures index, it is better to get an average number of transactions of all funds in a day, which is easier to estimate, and then divide this number by the trading volume of the day, multiply it by the average trading price of the day, multiply it by 300 yuan, and multiply it by 12%.