My 2 cents.

Choosing either cold or hot plug is about how the catalytic reaction taking place that relate to what the manner you tune both the L/H speed needles and how many the nitro content of your fuel is (the methanol content should be the case indeed). I rarely take ambient temperature into consideration for choosing the plug, maybe the climate here at my place is about average and lowest temperature in winter is about 7C. Rather I will adjust the needles according to the climate change upon each race day. However, plug with wrong length may never get the right tuning and make motor running weird.

1mm opening on the slider for idling is more or less an idication about what possible setting it should be. Leaning out either or both the L/H speed needles will cause the motor reving high at idle.

You can still measure roughly the temperature of your motor by spit or water test without the need of a temperature gauge.

Getting known about the factory spec. of your motor is better for tuning reference. But you can still get it running in good order by having it tuned at proper manner so that you can do without the factory spec. I help my buddies to re-tune their messed motor from the scratch of releasing both L/H speed needles to richen side.

Hope this help