Depends on the batch size and gravity of the beer to be made by it. I usually pitch about half a slurry from a lager and a third of a slurry for ales, if the same volume and same ABV. If a slightly bigger beer, then half a slurry from a smaller beer...800 ml is a lot of slurry for an ale...I would go with maybe 500 ml of good, clean slurry for what you are proposing. If you have a fair amount of beer in your slurry, then that might increase the amount you need to use. I pour off the beer at the point I pitch the slurry. The slurry starts to lose viability at a rate of 50% per month or so (possibly greater for some strains). This is all anecdotal, so YMMV on these aspects, but generally what I use to estimate things.