Where to Play the Flat Smash in Padel: Parallel, Not Crosscourt
The flat smash is not a placement shot. It is a speed shot. You are not trying to find a clever angle, you are trying to make the ball arrive before anyone can do anything about it.
Which means the only thing that really matters is how long the ball is in the air. Pick the wrong line and you hand back the time you were trying to take away.
Play it parallel
Hit the flat smash down the line on your own side. It is the shortest route to their court, and shortest route means least travel time.
The whole value of the shot is that it gets there fast. Parallel is simply the fastest available line.
Shorter distance means it comes back faster
There is a second benefit that people miss. A parallel smash also rebounds off the side glass and back into play quickly and at a low angle.
So even when they do get a racket on it, the ball is on them again immediately. You are not just hitting fast, you are keeping the whole exchange fast.
Do not play it crosscourt
Crosscourt looks tempting because the space is wider and the target feels bigger. It is the wrong instinct for this particular shot.
The crosscourt is the longest line on the court. Every extra metre is extra time for them, and time is exactly what the flat smash is supposed to remove.
Longer distance gives them time to return
On the diagonal your smash spends noticeably longer in the air. That is enough for a decent defender to read it, get behind it, and put it back.
You hit your hardest shot and they returned it. Not because the smash was bad, but because you gave them the extra half second on the way there.
Same swing, different line
You do not need to hit the flat smash harder. You need to hit it shorter.
Parallel every time, and let the reduced distance do the work your arm was trying to do.