For consulting I have a variable hourly rate.
Cost per hour
Development < 1 week
Development > 1 week
Development / Support out of hours
The cost a base rate which is $100 per hour plus a surcharge which reflects the level of responsibility involved.
When I provide a quote I often provide time range estimates for example, an item may look like this:
Enable push notifications: 1 - 2 hours
That means that for this work I would charge a minimum of 1 hour and a maximum of 2 hours.
If the work took 1.5 hours I would charge for 1.5 hours. If it took 3 hours I charge for 2 hours etc...