You need a good union or good negotiating powers. I too fell foul of this abuse of part-time. I worked a 50% roster in days of duty, but because I did no SBY's I achieved >60% productivity for 50% pay. After I left the said abusing outfit a union arrived and 50% roster was just that. It was a shared roster but the total FTL for any period was maxed at 50% of the limits and they received 50% pay. I was the pathfinder and, as often is the case, got shafted. I have mates in BA, KLM, who work a union agreed 50%, 66% 80% roster for the ratio rewards.
If you allow yourself to be shafted by doing 100% work for 80% pay then.............