I haven't used Foca or Roma in several years, but keep a supply of Ariel powder.
I never measure Ariel (or Foca or Roma) directly to a target dosage. I use a 50cc (3.38 tbsp) scoop, start with half to 1-1/2 scoops (~1.5 to 5 tbsp) depending on the load size / water level and as related to which machine is being used. Check the water for slipperiness after agitation gets mixed-up, add more half-ish scoop at a time until it is slippery (plus STPP).
I sometimes add a bit of Ariel to original Persil 2in1 liquid or Power Pearls (not much left of the supply bought from Target in July 2017) or Cheer powder (just opened the first of two remaining boxes) so as to conserve them which attaining suitable slipperiness.