I would not use the celery salt
I would use celery seed and grind it to a powder, then add a tiny amount if I added any at all. I do not care for the celery component in beef rubs at all. If I assume you are going to use the celery salt no matter what...1/2 part celery salt and 1/2 part salt. Celery salt is mostly salt anyways.
Although I use commercial rubs, if I am making my own, I want to control each element, thus, I do not use combo spices and herbs, I add what I want of each. Salt and pepper are the most important elements of this.
A very simple rub for brisket
3 parts Redmond Real Kosher Salt
2 parts black Pepper, ground to a medium grind, no fines
1 part each of organic onion granules and garlic granules
1/8 part Natural sugar, finely ground