It would be helpful if Billable time could indicate that it was only partially billable instead of splitting the entry into two timeslots, which is artificial.
I worked on that task for that amount of time, and while it is billable, only a portion of that time should be charged. To make it easy to enter, this could be done as a percentage. The default would be 0% discount. The discount% reflects the Non-Billable portion of that timeslot.