There are two ways to handle it in the minion.
#1 you can use the drop down box and select % of parry. Which strictly just gives you a -10 penalty, -20 penalty. ( no it doesn't work right by % it work more by difficulty)
or
#2 te way I do it is just use the Box on the left to enter the Value the player is deducting from his Ob to use for parry. If they player is using 16 of his 38 ob (50%) to parry then i enter -16 in the box on the left rather than using the drop down.
I much prefer to enter exact values instead.