Instead, you can punctuate or not punctuate, create your own style guide if necessary, and get on with your writing.
So here are some general guidelines that will get you through punctuating abbreviations and acronyms.
Guideline 1: Make an arbitrary decision, right now, about US/U.S. and UN/U.N. It is a coin toss.
Guideline 2: Are you in a scientific/technical field? Or is your writing for a more general audience? Pick one, and then go to the matching section below.
Guideline 3: Abbreviate units of measure only when preceded by a number. And even then, only if they occur a lot in your material. If you abbreviate units of measure, use lowercase letters and periods.
We asked how many feet the water rose during the flood.
They installed a 10-in. border around the flowerbed, which was a 3 ft. by 12 ft. area on the south lawn.
Guideline 4: If it is only one capital letter, use a period.
Guideline 5: If it is all capital letters, no periods. If there are any lowercase letters in the abbreviation, use periods.
Guideline 6: Make an arbitrary decision right now:
am and pm
a.m. and p.m.
AM and PM
Guideline 3: Abbreviate units of measure even when not preceded by a number. Don't use periods, except with in. for inch, because it matches the word in.
Guidelines 4 and 5: Almost never use periods for any abbreviation. See your style guide for exceptions.
Guideline 6: Why aren't you using the 24-hr system?
Standardize the abbreviations in this paragraph. Use either the general audience or the scientific audience rules. Caution: there may be other stylistic issues besides the punctuation of the abbreviations.
The flowerbed should be 12 feet wide and 5 ft deep. To determine how much sunlight the area receives, start at 5:30 AM and observe the area at 1-hour intervals until 8:00 p.m. I prefer to use the guidelines published by Ms Good Gardener in her blog, but you may want to consult others if your garden is outside of the US. Also be aware that some plants are sensitive to U.v. radiation. You can find additional information on the amount of sunlight your lawn receives by typing your Z.I.P. code into a search tool. If your garden is big enough, you may need an ATV to get from one side to the other with all your tools. FYI, yours truly once had an RV and an S.U.V., but now drives a Prius.