Help with scaling/ratios, expr!

Hi,

I'm designing a patch in which the length of the waveform~ display changes according to the speed of the groove~'s playback. I'm using a float connected to the sig~ object, as standard, to manipulate the playback speed, but I can not figure out how to use the expr object in a manner which works for me!

I basically want to say, for example:

When playback = 1, no change;
When playback = 0.5, *2;
When playback = 2, /2.

I want it to be adjustable to all numbers, not just to the above ones.

I know that it's not too difficult but my math skills are poor!

Thanks : )