How do you know how much "Padding" an index should have?
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If you're referring to the fill factor of the index, the only legitimate answer that you can get is "it depends." The fill factor of an index can be varied based on how much you'll be inserting into the existing leaf level pages of the index.
It's going to be best to create your indexes using the default settings and monitor the level of fragmentation over a period of time. You should also be routinely performing index maintenance using an index maintenance script. These scripts will automate the process of keeping your indexes in good operating condition.
I believe that "fill factor" is overrated.
I believe that you should leave room for one more item. That way, every block is ready to receive one more row without having the overhead of a block split. As you proceed to do stuff, block splits will happen. If things are random, and one block turns into 2 (not something more sophisticated), then eventually the average block will become 69% full.