If you are using RO water and mineralising that, you can just use the TDS pen to measure and all the 'others' will be in balance.
In basic math, 1 scoop/10L will mean 80% of a scoop in a US 2 gallon (1US gallon is 4L), if it has 2 gallons of water, and it would probably have less than that unless there is nothing else in it and it is filled to the rim. I would try a little over half a scoop to start with as it s easier to add a bit more than reduce it! Make sure it is well mixed/dissolved before taking any readings! There is a range with all figures so don't fixate too much on exact numbers, just aim for those?
When changing water with the shrimp in situ you should add the new water slowly, usually some sort of dripper is best! I assume though at this stage you are more focused on getting the tank ready for the new batch en route rather than the few that are left and probably weakened anyway?
Simon