I recommend using a Code field instead which helps reduce the number of queries your site has to make. Also gives access to the many math functions PHP has built int. Example as follows using a Code field provided by CB Code Field.

Template: Same as Global
Code (PHP):
Now for this to work on the registration field you need the field to refresh its display when any other the other fields values change. To do that you'll need CB Core Fields Ajax. Once installed and published edit your code field and under Integrations > CB Core Fields Ajax configure the following.

Update On: cb_FIELD_1, cb_FIELD_2, cb_FIELD_3, cb_FIELD_4, cb_FIELD_5
Display Empty Update On: Disable

Query field won't work in a similar situation during registration since the user doesn't exist yet so there's nothing to query.

This is our ajax call

What do you mean by this? You're making your own ajax call? From where and for what? Your code looks fine. The problem is likely how you're trying to access the field.

Ok, was able to confirm the issue. It's due to our CBuser API not caching the guest user object and is rebuilding it on every request so during registration the data set by CB Core Fields Ajax is just lost since CB Code Field is then building a new guest CBuser object for substitutions.

forge.joomlapolis.com/issues/8796