Looking for advice..much water do u add to masonry?

Hi

Got outside masonry to paint tomorrow but to be honest everytime i do one i never get mix right and end up using tons more paint.

Its been painted before and is now going a gray colour.

Could u tell me how much water u would add to a 10 litre tub? Just so it goes on easier and goes further plus its getting done on a Saturday a want easy shift haha.

Thanks in advance