You should follow the guidance and use a candy thermometer to kill the nasties.
Once you make a few batches, you can usually eyeball it, different milks will act subtly different at temperature. Heating also changes how milk components can consumed by the cultures. I get milk from a farm that doesn’t homogenize it the same way as store stuff - the skin develops on the surface sooner.
Personally, I prefer to use a yogurt maker that keeps it at a consistent temperature. But you can make great yogurt in a variety of lower tech scenarios.
If you happen to be in the UK - Graham's Dairy milk (sold in supermarkets) is 5%. But you're right that 'whole milk' is 3.5% or greater, typically 3.7% for supermarket own brand. Gold top or Jersey is 5%.
In Finland they sell some small portion-bowls with a thin solidified cream layer on top. It's not yoghurt but something we Swedes call "Långfil" [1]. It has a very funny consistency, it doesn't stick to the spoon as normal youghurt and is very delicious!
Once you make a few batches, you can usually eyeball it, different milks will act subtly different at temperature. Heating also changes how milk components can consumed by the cultures. I get milk from a farm that doesn’t homogenize it the same way as store stuff - the skin develops on the surface sooner.
Personally, I prefer to use a yogurt maker that keeps it at a consistent temperature. But you can make great yogurt in a variety of lower tech scenarios.