Hello Thomas ! I am happy that you found this useful.

The values of m and c I got are straight from the jupyter notebook. You can find it here: https://github.com/chasinginfinity/ml-from-scratch/tree/master/02%20Linear%20Regression%20using%20Gradient%20Descent

And its a very interesting thing you did there. Everything you did is perfectly correct and the values are also correct. I prepared this colab to help you understand what exactly is going on, and why the values are not what you expected

Google Colaboratory

Check it out and let me know if you have more questions or clarifications :)