Per Diem, is by original definition a payment made to crew members to cover meals and other out of pocket expenses when on the road away from base over night. Per diem while operating to/from home base.... not likely.
If I flew from Berlin to Milan for the day and back that night then no per diem, if we did a full day 6 legs and ended up back home in bed, no per diem.
You cant have it both ways, general rule, if you sleep in your own bed, and you are salaried..... thats it.
If you dont sleep in your own bed, at the behest of your boss..... its on his account.
This all got mucked up when companies started using contractors instead of having full time crews, and then started to screw them on the day rate.
As a professional, your day rate needed to cover your income, tax, overnight expenses, recurrent training, etc. If your day rate did not cover your expenses then you are not charging enough.