We had to mix feed DS2 which was new to me so I did get quite obsessive over the rules, then I relaxed a bit and realised most people don't follow the majority of them and some of them are common sense things you'd do anyway.
Essentially you want to do two things - manage contamination from the environment, which means things like:
Wash hands (especially if someone is ill)
Keep kitchen clean/don't make up formula in a dirty environment
Try to avoid anything falling into the tub
Use clean/dedicated utensils to make (e.g. clean knife or the purpose built leveller in the tub)
Clean old milk bottles well
Sterilise the cleaned milk bottles in case any remnants of milk were left after cleaning
Protect tub from moisture, excess heat/cold/sunlight etc
Discard open tub after 4 weeks (also due to potential vitamin degradation IIRC)
Discard feed after 1 hour when baby's mouth has been in contact with it
Keep lid on bottle to protect teat
And then understanding that if the powder ever does get contaminated, which is possible from the factory itself (though very rare) - that milk is a very welcoming environment for bacteria and when it's kept in the "danger zone" (5-60C) for any length of time, any bacteria present may grow to dangerous levels. So reducing risks of this:
Make up with hot water when possible (over 60) especially if you will be storing it.
Feed ASAP after making up, esp if made with cooler water.
If cannot feed immediately, cool feed and keep in fridge (under 5C)
If taking pre-made milk out and about, use a cool pack
Stick to rough guidelines on time:
Max 2 hours at room temperature
Max 4/6 hours with a cool pack (I forget what this is,NHS has the guideline)
Max 24 hours in back of fridge
After this time has elapsed, discard the feed.
Discard any feed which smells or tastes sour or has curdled.
Recognise that the risk is small to begin with, and is most serious for babies under 4 weeks old. Older babies have more defences against food-borne illness.
All of the bacterial growth rules are likely overly conservative given that until 2001 the advice was just to treat made up formula like fresh milk and that it could be kept at room temperature up to 24 hours. I'd still stick to the up to date ones, but it helps add some perspective.
Likewise in many countries making up with hot water is not considered necessary, and neither is sterilising.
Most of the guidelines are a kind of "swiss cheese model" multiple safety nets in case some or all of the others aren't present. So it makes sense to do as many of them as possible, but at the same time, no individual guideline is critical because there are so many other safeguards to begin with and also the chance of it being contaminated in the first place is low.