I have an excellent example this morning of what we do in Margie Rules. This is my $10-a-month subscription program. We offer a 10 percent discount on all my books and seminars; a live monthly session online for questions and discussion of English topics; and a dedicated FB page, where I answer questions and explain the grammar/punctuation that applies to the asked-about section.
The section being asked is the opening paragraph in red below, followed by my explanation in blue of various issues that come up.
…Just reminding the Court that there is a lot of evidence that apportion of whatever was done in the two to three hours a week or four a week has nothing to do with tree trimming — excuse me — shrubbery trimming and mostly to do with the lawn area — half of which or maybe it’s not half, maybe it’s a third. You saw it; you will determine — on Sam’s property, de minimus.
…Just reminding the Court that there is a lot of evidence that a portion of whatever was done in the two to three hours a week or four a week has nothing to do with tree trimming — excuse me — shrubbery trimming and mostly to do with the lawn area, half of which — or maybe it’s not half; maybe it’s a third. You saw it; you will determine on Sam’s property de minimis.
There is a lot going on here.
It is “a portion” as you need a noun in that place in that sentence, not a verb. “De minimis” has an “i,” not a “u.”
“Half of which…” needs just a comma as the word “half” is an appositive to “area,” which is right in front of it. “Half of which…” is right where it belongs. Then the dash is where he starts over/corrects himself with “or maybe….”
“Maybe it’s…” is a sentence unto itself and needs at least the semicolon, which you would choose because of the parallel construction.
“On Sam’s property” could take commas around it, but I think it is just part of the sentence and doesn’t need anything. It is just a prepositional phrase.