273 questions...a few of them

If you set it according to the chart you will be okay. This eliminates the factor of overlap in timing. Anything else is just a guess. I'm not saying it won't work w/ other methods, but it beats fining out the hard way.
It really doesn't matter which way you insert the feeler gauge as long as it's positioned between the rocker arm & the valve tip evenly. The amount of drag is more critical since too tight or too loose will not give you the proper clearance. I would suggest practice by getting a feel for the correct amount of drag on the gauge by putting the gauge in the jaw of a micrometer adjusted to the gap or lash amount you want & pulling the feeler gauge through it to get the feel for it. In other words set your micrometer to .021" & pull the feeler .021" gauge through it. Same thing for the .013" gauge, mic set @ .013". You shouldn't need to hold the rocker in place because once the lash is set w/ the feeler gauge in it there should be no clearance until you remove the feeler gauge.