Across the Board Budget Cuts - Fair for everyone?

Recently our nonprofit organization announced "Across the board budget cuts" and has asked everyone to reduce their hours and pay by 25% "across the board." This is a valiant effort in order to avoid layoffs, which our small firm would not be able to handle.

However, how does 'across the board' work with part-time workers? Some of our staff already reduces hours every summer voluntarily for their personal/family reasons. A 25% cut for them would be what they would normally do anyway. Whereas for full-time workers, it is a significant cut in pay and hours. What is a fair way to administer across the board cuts?

How do other agencies, companies, etc. make that happen?