Match the data type of the field you create to the field you are relating to.  Don't create a Float related to an Int, for example.

Never create a related field based on (has a dependency of) a field that itself is related or computed. 

If you don't need to store the value, don't. 

Adding a stored related field on table where you already have thousands of records may not be possible, as values for all existing records need to be computed first, and you may get a timeout.

Make sure to avoid circular dependencies.

Review https://www.odoo.com/documentation/12.0/reference/orm.html#related-fields