It's great to hear how you are using NEXUS and very interesting to see what combinations of inputs people gravitate to -- I hope to see some of your gameplay in the future!
And congratulations on being the first person on the forum to get to this level of customization of the triggers to be hitting the issue you describe.

Below is a page out of the
User Guide and is a good visual for what needs to be done in your scenario. Remember that you are mapping a NEXUS Trigger to the Console Trigger. The bindings you place specify points along the press to match up. So, as you are pulling to 70%, the Console Trigger is going from 0 to 100% (where it hits 100% when NEXUS Trigger hits 70%). What you want to do a specify the 0% Console Trigger position at say, 65% of the NEXUS Trigger. Then your Console Trigger (called "Game Trigger" in the picture) goes from 0 to 100% when your NEXUS Trigger goes from 65% to 70%.
You'll also want to be sure to tell XIM NEXUS that you are firing using the left trigger by specifying the "Fire" role when the binding that causes firing happens.
Please let me know if you need any other clarification.