Commissions: some clarifications

I would like to make some clarifications about commissions.

This is the process i follow when i accept an commission:
- Make One or more sketches with the customer's idea.
- Customer may ask any changes on the sketch.
- The sketch is confirmed (here i ask for payment)
- Proceed with pencil
- Proceed with ink
- Proceed with fix ink at pshop
- Proceed with color & delivery.

I'd like to clarify a few things. Usually during sketch i like to fix required corrections can happen because there are some misunderstandings of  differently language spoken, so often it can happen that i don't understand the type of request, or misrepresents a few requests.
When the error is mine, i can do dozens of changes on a sketch.
But i can't accept one thing anymore: the continuously change of idea on the sketch, which, sometimes, is pursuing the work for months without being completed. I can't accept this.
It can happens that the customer wait some weeks before i can begin the work, but, once when i start, i finish in a few days. So i need to define the sketch as soon as possible, so i can deliver the commission really in a few days.
Spend weeks, or months, only to change sketch is unnerving, it makes me lose time and other possible works.

So, from this day forward, i decided to fix the sketch up to where i am in error. Also i accept change idea, i will not do more than three changes. Otherwise, i'll ask an higher payment.

Most important of all: Once the ink or color is complete, it is NOT possible to request other changes.

After so many years i have made this decision, i hope this can't offend anyone. But i need to manage my time better, and more importantly, lose less money as possible.

I hope you can understand!