Hi Jim
Put in a cup, let the agitator swirl, feel the water, and if it's nice and slippery, you're good to go.
Back in the day, the standard dose was 1 & 1/4 cups.
If Ariel is a newly concentrated version with a scoop enclosed, just try a scoop.
The Error consequences here are slim to nil, because if you over-suds, then YAY, it's a holiday, and fireworks have begun in Jim's wringggggger washer.