Part time in my co was granted Iaw co needs, so you could not demand it. Contracts down to 50% were given, the notable feature being that it was a percentage of days worked In a month for that percentage of salary, not hours on duty. Other benefits that were divisible, such as leave, days off and pension were also reduced. Rostering would then busily try and fit as much work as possible into the working period! I was on 75% and enjoyed having a predictable 7 days off for the foreseeable future. The is also a gain insofar your tax bill is reduced. The company still has the full costs of having you on the pay-roll such as administration and simulator/training costs, but it has a reserve of pilots who will probably work overtime on request. I always made up my lost salary working days-off but at my convenience.