When itemising your invoice, although it isn’t possible to select anything less than whole numbers when adjusting the unit amount with the ‘+’ and ‘-‘ buttons;* there is in fact way around this.
If, for instance, you are charging for your time (with an hourly/day rate), manually calculate your total cost for the time you’ve worked, and then specify your hourly rate or a breakdown of your calculation, either within the item description, like so:
Description: 60.5 hours x (hourly rate = £10.50)
See examples below:
Specify it elsewhere on the invoice, such as within the additional ‘comments’ field (accessed via ‘More’ at the bottom right-hand corner of the invoice creation screen).
See example below:
Below are examples of how either method will look like in the final invoice. On the left: if it is included within the line item. On the right: if it is included within the additional 'comments' section. Choose whatever method you prefer or find more suitable.
*[Please note: the units column only reaches 99]