Adjust multiple align to spline values

Hi,
tried this in Xpresso math node but not getting far.
I’m positioning several objects at various points along a long spline using an align to spline tag. I’d like to reposition them together (i.e. increase/decrease their position % values) but keep the relevant distances between them. Is there a way to do this - kind of like having a master value I can increase by 2 or whatever and all the individual values increase by 2.
Hope this makes sense. Its kind of like having an audio bus I can adjust several values at once.
While I’m here, wish align to spline had finer increments than 0-100. A long spline means tiny wee adjustments.
Anyway,
thanks for any advice.