Shift leaders earning only 4% more than the operators they supervise: what gap is normal?

After the latest collective agreement increase for operators, our shift leaders are on average only 4% ahead of the highest-paid operator on their shift, and that is before overtime. With overtime, which leaders do not get, several operators earn more.

This is a chemicals site in Belgium, about 650 employees, with operators under the sector agreement and shift leaders on the company's salaried structure. We have had two leaders ask to step back to operator in the last six months.

What supervisory differential do you aim for, is it measured on base or on total cash, and how do you maintain it when the two groups have different increase mechanisms?