Your waiting time charges should be configured in your rate cards.
It should be entered at an hourly rate, when you then apply the waiting time you can select the minutes you want to apply and it will divide the cost accordingly.
You can then set on a customer account the number of minutes waiting time that they are not charged if applicable.
Within the account under edit details > Rates / Frameworks
You then apply the waiting time to the job by selecting waiting time, select the minutes waiting time you wish to apply and it will then apply the applicable charge.