You need to use tick offset with percentage option, but it wouldn't exactly work out how you want. It will however in the latest Beta version. See
http://gruss-software.co.uk/forum/viewtopic.php?t=3922 for download link.
The first time you login to the new Beta version go to general preferences and tick "Calculate tick % on decimal odds". You only do this once.
Before you place your back bet select the tick offset options tab, enter 50 in the "tick offset" box, tick "auto level" and tick "ticks = %".
Don't forget to disable tick offset after placing the bet as it will apply to every subsequent bet you make. You can disable it quickly by clicking on the green tick next to "tick offset" at the top of the screen. It will change to a red cross to indicate it is not active.