This will depend on the specific roast, dose, and freshness of the beans being used, as these factors all affect the density and weight of the bean and resulting coffee bed.
If using one of our standard models (NEO-PRO2), the grind size will be coarser compared to the size required for the Flair 58, as these models have a more narrow-yet-deep basket. It will take some experimenting at first to "dial-in" your ideal grind size, and will require small tweaks if changing beans or other variables.
If no/low pressure is achieved, grind finer until your are hitting the target of 6-8 BAR for 40-45 seconds (or 6-8 BAR for 25-30 seconds on the Flair 58). Adjust to taste.
Remember, if you can’t lower the lever without a significant struggle, you’ve ground too fine. In this case, carefully and slowly raise the lever (to avoid a rebound effect) and restart with a coarser grind size. Do not attempt to force the lever down and never brew in the "danger zone" on your gauge - this may result in a breakage.