Holiday default

We have noticed that if an employee is part-time with no work pattern (which many of ours are as they work on a rota basis), the system defaults to 7.5 hour days when booking holiday. As our standard day is 8 hours, this is causing issues with employees overbooking holiday (ie 7.5 hours deducted from total rather than 8, so they think they have more hours left than they actually do)