Whenever I do pastrami I love to use some black pepper and coriander that I toast in a pan before grinding up, some garlic, ground bay leaf, mustard seed and a touch of brown sugar, and chipotle but I put that in almost everything.
I don't generally measure but for a 5lb corned beef I would probably go
bay leaf is optional
1/4-1/2t mustard seed (not ground)
1/4t brown sugar
Give it a good coating, you want a nice layer of peppery rub to tantalize the taste buds.
I do find toasting the pepper and coriander add a wonderful depth of flavor the the finished strami.
Never added old bay, could be good with the flavor I suppose but I like to keep my pastrami more on the traditional side (with the exception of the chipotle)
Hope any of this was helpful, and good luck.