I think you've more or less got it (not sure about "on-target"). To paraphrase what I've understood there, any salary adjustments are calculated using what the employees would nominally earn when they are working their full working time contingent.

I assume in this case that "bei vollem Arbeitszeitanteil" means that a part-time worker that works 20 hours a week will have their pay adjusted on the basis of a full 20-hour week (as opposed to a full-time worker working a full 40-hour week, for example).