Here is the simple truth. It takes seconds in a food processor, and only minutes by hand, just make sure everything is cold: chilled butter, hands, slab, and work speedily. Then chill the pastry. 30 minutes is fine, longer and it might be too tough to handle straight out of the fridge. It depends on your fridge, mine is American and brilliantly, aggressively cold.
If you are making a 20–24 cm tart, 180g flour to 90g butter works best and you may have a little over for an individual tart or patching or lattice.