I wonder how correct it is necessary to organize work on a long-term project in the case of hourly?
To begin with - go to a real hourly rate.
This is when you first work then you get paid how much worked.
If you first said 8 hours is approved, multiplied by the hourly rate and got a figure which will pay you for the task - where is the hourly pay? it's just another way to justify the fixed price.
So you worked for a fixed payment and work.
For schemes with a fixed payment, to avoid such problems in the first place we must learn to assess the task correctly, as well as to assess the risks (potential inaccuracy) and all that lay in price.
As long as you experience little or evaluation if the problem is quite new - it is a big risk, so I appreciate the task in 8 hours, the customer say it will take 15-20. Then look at the fact how occupied and learning.
If you say 8 and it takes 2-3 times more, and it happens all the time, it is obvious that you greatly underestimate the time estimation.
Why is another question.