Hello Mahlon,
So far if you are creating something New All together like a New Item or New Invoice we are calling those "New / Edit"(New / Edit Invoice.
If you are adding an items to something like a line item to an invoice it is "Add / Edit"(Add / Edit Item).
The reason why estimate has New / Edit Estimate and Add / Edit Estimate, is because there is no is way to delete screens that have been pushed out as part of apps.
-Inder