hopping and thumbs

I've been practising hopping onto narrow kerbs and am still learning to control
exactly where I land so I don't bounce straight off. My major fear at the moment
is that when I don't land correctly, the uni bounces back up at me and can twist
my hand around. When holding my viscount saddle, I have four fingers under the
rim of the saddle and my thumb pointing back down the seat. The thumb is the
problem, because if I misjudge the jump too many more times, I'm going to break
my thumb. How do people get around this but still retain control of the saddle
when jumping onto small pointy objects?

Thanks, nic