Learn English – “Add up to total” vs “add to total”

In general, if you have a number as an adjective, such as five percent of cases go unsolved, 76 percent of them voted in favor, or 50 percent of coin throws land on heads, you use percent of.

If you have roughly the same percentage of men and women, a low percentage chance of failure, or a policy meant to decrease the percentage of drivers without insurance from 25% to 15%, percentage of is more common.

So, in your example, I personally would use frequency as a percentage of the total in a sentence. The other would be perfectly understandable too, and I may favor it (not sure though) as a graph title.

The rules you were taught are artificial. It is very rare to hear "as I" used in the way you have it in your examples out in the wild. It may be correct according to prescriptive English grammar, but it is not idiomatic to the language until you add the extra bits. One would say either:

She is smarter than me.

or

She is smarter than I am.

The same pattern emerges when looking at like. Used by itself, in idiomatic English, you would use the object pronoun when the word is used alone, and the subject pronoun when the phrase extends into a sentence-like structure.

Despite the vain longings of those few people who want English to be nice and neat (and have a one-to-one correspondence with Latin and Greek), our language has its wrinkles and inconsistencies. This is one of them.