You can turn off the Validation of calculated fields when clicking the Save button by running the following query:
INSERT INTO Configuration
VALUES ('1', 'SYSTEM', 'VALIDATECALCONSAVE', 'FALSE')
some people want to do this when creating a lot of the same calculated field and the validation is taking many minutes.
But listen carefully, the steps to make it work are not intuitive, here they are:
1) Before you create your calc field that you don't want validated, save a calc field that has the correct data type for what you intend your final calc field to return. For example, save a calc field whose formula is 1 (i.e. integer).
2) Then edit that calc field you just saved and delete the formula of 1, and enter your intended formula, this will then save without validating. You can easily test this out with a formula like SUM('xxx')
(If you prefer videos to reading, please see the attached 2 minute video for a demonstration of the above steps)