Excaliber,
I agree with your concerns. If you do all 16 valves at once, the engine will have cooled a lot by the time you get to the last valve. I suggest taking your best guess at the cold lash setting. Set all 16 valves cold. Then take the car out for a good long run allowing water and
oil temps to come up to normal. Get it back into the garage and as quickly as possible recheck the lash on just a couple of valves. This should tell you the difference between your hot lash and your cold lash. Let the car sit and cool overnight and then the the next day you can reset all the valves to the newly calculated cold lash.
Of course you would want to always set your cold lash when your engine is at roughly the same temperature. That is, if you use the above method to find the proper cold lash on a 50 degree day, your results would be different than if you did it on a 90 degree day.
Chris