I think you mixed the reagent incorrectly. I made the same mistake initially.
"Now carefully fill upp with reagent to the readings of exact 400g.
• DO NOT remove the canister from the scale, let it be, still reading 400 g of
course, and now add RO-water until you read exact 2000 g at the scale. Done!"
So it should be 250ml and then fill the rest to make it an even 1000ml which would be 750ml water.