* it must be reasonable from the user's perspective
* there must be alternative; you cannot achieve the goal (your "legitimate interest") without it
* it must be balanced with the rights of the user, and not infringe on their freedom or fundamental rights
* if your "legitimate interest" is direct marketing, the user can always object, and you are required to actively inform the user of this right
See also [1]
If you can provide a service strictly devoid of the PII it means there is no logical necessity for PII.
You can't provide a call-waiting service without a phone number, but you can provide a mail-redirection service without one even though it makes it easier to administer when you have a customer phone number, you can strictly provide (and bill/administer) the service when that information is absent.
While I agree with your comment, I suspect that for any given law firm, paramTotalHours_Billable(SubjectID = necessary) will be a much larger value.