I used this for a year without any issues with my 2.2kW spindle:
It had 9l of antifreeze in it. 20l as Luke says will give you even more thermal margin.
EDIT: latest news is that spindles may like to stay under 35°C, and with those 9l of coolant with no active cooling it takes about 5 hours under light (but continous) load to reach that value.
side note: being from the old world I had to look-up how much a “quart” is, and google says it’s 0,946353l, is that how much a “quart reservoir” would hold ?