NX doesn't automatically adjust gaps when bending flanges...
I don't believe the answer is just so straight forward. The bending is different for each machine. And also corresponding to the Tools used in the machine. Not sure about how it's call. But I believe it has to do with "knife" used. Sharp/blunt. And the side where it's pressed into.
Maybe others can give you practical tips/tricks of the real life
If you want to be very exact with this desired dimension you will have to;
-Use exact bend data (Bend Radius and Neutral Factor ) As Dennis already mentioned this will be depending on:
-Unbend the specific bend and manually add material with e.g. a Secondary Tab. Then rebend. That way you will be 100% sure that the gap will be what you want it to be in the flat pattern.
BTW: In your drawing the horizontal gap is driven by the Bend Relief Width.
When working with the actual bend radii (that reflect reality) the bend relief DEPTH can always be set to zero (0); The bend relief depth is there to prevent that an inset bend radius will tear his way into the adjacent tab. This "tearing" will be more visible with thicker and stronger materials.
I hope this helps.