During the first year new employees are granted 5 days of PTO accrued at the rate of 3.333 hours per month and they are allowed to take no more than 20 hours of PTO during the first 6 months of employment. It is not much but it does help.
We watch the PTO closely and we have at times allowed employees to exceed their accrued PTO and only once have we had to "eat it" in the last 15 years. As we pay weekly we notify the supervisor if the employee is over his accrued time and then let the employee make the decision whether to "go into the hole " or take off without pay. Although it is not in our policy we do not allow them to go into a deficit of no more than 3 days and if they do we tell them they will have to take further time off without pay until they have accrued enough to cover what they have already taken.
I think that with our policy of sending out their PTO account balances every other month helps them to understand where they are in the use of their PTO. We do encourage employees to call in to find out what their PTO balance is before they take time off so that helps also.
Hope this gives you some idea of how it may be handled.