Professional players clearly understand their potential and limitations.  They know how hard they should loop, where to loop, when to loop, and when not to loop.  Ma Long loops most of his balls with 60-95% power.  His selection on how hard to loop depends on his positioning, his distance from the table, his opponent’s return, and his opponent’s positioning.   Timo Ball loops most of his balls with 40-80% power; this is the zone that he feels most comfortable playing.
 
Now, let’s move the scenario to YOU…
You have a played a wonderful match looping consistently from both forehand and backhand.  You are playing an opponent that you have never beaten before.  You are at 2-2 in games and now you have match-point at 10-9 in the fifth.  You know that you should attack first.  How hard should you attack?  Within your zone!  Know your limits and do what you do best! 
 
If you loop too hard, you might lose control and make an unforced error.  If you loop too soft, you might lose control because your opponent’s backspin push will bite into your rubber making it more difficult for you.  Swing through the ball, complete your stroke, have confidence in your loop, and treat it like any other normal loop.  Remember, you need to have some racket-speed in order to generate spin.  Spin gives you control, which makes you more consistent.  Consistency is one of the main keys to winning every match!