Hi,
No, that's not correct. Custom item fields of the type "hidden" can also be used in the formulas.
However, the same rules apply to them than other custom item fields. For the field to be used on the frontend, it's frontend display setting needs to be activated.
This also means that the user can potentially change its value on the frontend. So basing yourself on it for the calculation is not recommended.
What you should rather use is a custom field of the table "product" and of the type "text".
You can still configure a default value in it. And when you create a product, that default value will be set in the custom field.
And if you want to change the value for a product, you can edit the product.
And if you need the value to change for all the products, it's possible with a mass action.