Unfortunately, automations can’t currently perform calculations. They also can’t perform conditional statements, although you didn’t ask about that. :slightly_smiling_face:
If you could make your desired value the result of a formula in a formula field, then you could update the number field based on the formula field.
You could also write a custom JavaScript to calculate the value as well.
One interesting thing to note is that the automation platform Integromat DOES allow you to perform math & other transformational functions on your data.