This is very important.
I thougt we can update both custom value and and custom fields using math operations but you can only use custom field with math operation in workfow.
This is going to solve a lot of problem and minimize using 3rd party integration.
In this screenshot you can only update standar field and custom field. Custom value is not available. https://www.loom.com/i/5d4e3dafa2ac4d19ae28066ff698aef3