Two at a Time Baking

I just made a batch of these cookies but this time I cut the salt back to 0.75 tsp and then sprinkled sea salt on top - a definite improvement!  I am still on a mission to figure out the best combination of variables that results in the least amount of spreading and least dark cookie bottom.  Tonight, 2 of the 4 batches came out really well and I can't quite figure out why.  Here are the variables that I am currently contemplating:

• baking sheet vs. jelly roll pan
• parchment paper vs. silicone baking mat vs. greased with butter vs. ungreased
• low vs. middle vs. top oven rack
• whether and when to rotate the baking sheet

I guess I'll just have to do more baking, all in the name of scientific research!