No matter what type of one handed backhand you are hitting, at the very least you need the thumb behind the racket, to withstand against the impact of the ball on the strings and put strength behind your swing.
Look at Pete Sampras in two magnifications of the same picture (below) and also a rear view of Tim Henman in the third image.
See how the thumb is behind the grip?
This grip is good for flat drives, hitting down on the ball for slice and moderate topspin.
However, to master both slice and ripping topspin on the one-handed backhand you’ll need (at least) two different grips to fund the two different shapes of backhand.
But we’ll deal with these soon.
In the close-up of Sampras you’ll see that the thumb is parallel to the back of the grip/handle, which puts the strength of Pete’s arm some way behind the racket – similarly Tim Henman in the third frame.
Contrastingly, if you hit a one-hander with a good forehand grip, you’d merely be pulling the racket towards the ball.
Your arm strength would be the wrong side of the racket.
I call this the Tumb Theory (or even the Thumb Theory, when I type it correctly) which warrants its own section.