When I first did a drain on my 940s, I noted where the cold fluid was on the dipstick, drained it and then did indeed dump the old dirty fluid into a milk jug. I marked the level on the jug, then cleaned out the jug with rubbing alcohol and refilled it with fresh fluid to the mark. I then poured this into the fill tube. I used a measuring cup to fill the jug to the mark with water and noted the total quantity on the maintenance record so I know how much fluid goes in each time without thinking about it.
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See the 700/900 FAQ under 'Select Link' button on the top right.