I need to calculate the current date in relation to the start and end dates of a task and would expect to see a field that shows the completion percentage.
Something like this Question or this explanation
(today - start date)/(End date- Start date).
To calculate the percentage of a task that should be done based on the current date and working time (e.g. the project calendar), customize a number field with this formula:
IIf( [Start] > Date(), 0, IIf( [Finish] < Date(), 1, ProjDateDiff( [Start], Date()) / [Duration]))
Note: answer was updated to reflect OP's correction to handle future tasks.