Hi there @Brendan_Faulds and welcome aboard 
O absolutely agree that the addition of the “or” condition would enhance our app. We’ll try to consider this opportunity in the future updates 
In the meantime, we’ve found a workaround that helps to implement “or” condition in our Forms apps. It should work for the Calculator as well.
Feel free to check it out - Forms: How to show a certain field based on one of several possible conditions