I recently added a new option to the latest Beta version which will allow you to do this.
1) Make sure you have version 1.1.0.66l. See
http://gruss-software.co.uk/forum/viewtopic.php?t=3922 for download link.
2) Make sure the following setting is ticked in general preferences.
Calculate tick % on decimal odds
(You will only need to do this once)
3) In the tick offset settings tab enter 50 in the tick offset box and tick "ticks = %"
4) Place your back bet and the lay bet will be placed as soon as the back bet is matched.