There are a couple ways of handling this.
You can specify the Target on a specific net by using the Right Mouse Button on top of the Delta:Tolerance value field and select "Set as Target" from the popup menu.
You can remove or blank out the Delta value and just specify a Tolerance that all the nets will be matched to with no specific target set.
Setting both the Delta and Tolerance values matches the nets to a +/- of the Tolerance value to the Target but once you blank out the Delta number it will be treated as an absolute match value.
Example for Item 1: Setting the Delta:Tolerance field to 0.000mil:50.000mil means the longest net in the group could be 50mils longer than the target net and the shortest net in the group could be 50 mils shorter than the target. So across all the nets you could have a variance in length as much as 100mils.
Example for Item 2: Setting the Delta:Tolerance field to :50.000mil means that all the nets could have a variance in length as much as 50mils with no specific Target. This is sometimes harder to match the members but at least you don’t overly lengthen the nets.
Hope this helps,
Plexus Engineering Solutions