Yes, a X offset in absolute units would be a useful feature (i add it to the wishlist). Unfortunately, at this moment it is not possible to release a quick beta. We are now redesigning the plugin, adding drastic changes to the core. The next beta will take some time yet.
Anyways, the percentage of the Bevel's Offset is based on the size of the Default Segment, not in the spline. So, if you use the same piece for all the structure, that should not be a problem.