Start with the manufacturer's instructions. But if you have a slimline dishwasher and your loads are not very soiled then you may be able to trim the dishwasher detergent amount a bit, and save money and a knock-on effect on the environment. In particular, Mike Fell tweeted
Feb 2021
Bonus tip ... you can use half a tablet instead of a full one in a slimline dishwasher. For us as of 2021 the Ecotricity electricity for an eco wash (~0.73kWh) is ~20p and an Ecover all-in-one dishwasher tablet is also ~20p: the half-tablet trick saves me 10p/25%! It works well here.