do it once or twice more and the system will have all fresh fluid in it
Statistically it takes many flushes to get all the old fluid out this way.
Replacing the fluid in the reservoir only, and not the pump and rack, you're getting maybe 50% of the old fluid, so with the first flush, you get 50%, the second another 25%, third another 12.5%, etc. And with each subsequent flush you're taking out more and more new fluid, an expensive way to do it.
The ideal method is to combine the two. Replace the fluid in the res., and then open the hose and pump the fluid out, the second half of this would be new fluid from the reservoir. If I'd gone full term mileage on my fluid I'd do it this way, it would take buying another liter.