First - how big is the area to be painted? Thats to decide how much paint you'll be needing.
Second - if you haven't opened it take it back for a refund. This own brand stuff is horribly thin - 9/1 ratio for a mist coat indeed - because its mostly water anyway. Even with this weekends "savings" you'll be better off getting yourself a 5 litre can of Dulux Trade Supermatt for use on new plaster - because it will go further and a 5 litre can of this will cost you less than the 10 litre pot of t'other stuff. DO NOT BUY RICH MATT - it doesn't stick to new plaster!!
Third don't try to dilute it in the can it comes in - because I think thats what you are asking. Pour out what you want into a tray or scuttle then add water to dilute. For a mist coat with Supermatt I use a 25/75 mix. 2nd coat use Supermatt again (or whatever your finish is (Dulux Trade Vinyl Matt recommended)) diluted 20% water 80% paint and final coat of your finish only about 10% water.