I am using a liquid detergent and I use exactly the half of the detergent they say I should use. If a the washing machine requires 1 cup, I do half. Mostly because I don’t trust the company to put me over their interests.
Now, what will happen? Will my clothes end up staying dirty? Will it not remove stains which a full cup would have removed? Will surfexcel kidnap me and torture me for not obeying their commands? Help meeee!!
If you use too little, it won’t get your clothes as clean. If you use too much, your clothes will come out of the washer still with detergent in them or perhaps you’ll have issues with too much suds leaking out of your washer (or at least out of the tank portion potentially into some of the electronic components.) There’s probably a pretty wide margin of error, though, and you’d have to use a lot too little or a lot too much to see any noticeable difference, though. If you’re happy with the results you’re getting, keep doing what you’re doing. If you feel like doing some experimentation with the amount of detergent you use, hell, everybody needs a hobby.