You are in charge of your homeschool policy. As the parent, you can choose any grading scale that you want to use. If you are homeschooling independently, then you get to decide.
There are a few commonly-used grading scale options I usually suggest you consider. Which of these will strike your fancy?
Grading Scale Options
97-100% = A+, 93-96% = A, 90-92% = A-, 87-89% = B+, 83-86% = B, 80-82% = B-, 77-79% = C+, 73-76% = C, 70-72% = C-, 67-69% = D+, 63-66% = D, 60-62% = D-, <60% = F
A=4.0, B=3, C=2, D=1, F=0
93-100% = A, 85-92% = B, 75-84% = C, 70-74% = D, Below 70% = F
90-100% = A, 80-89% = B, 70-79% = C, 60-69% = D, Below 60% = F
There are other options out there, but there is no "right" answer to the best grading scale. You can choose the one that's easiest for you to use, the one that looks the most intimidating, or the one that helps you sleep at night. You could even use the one your friends, neighbors, or public school uses, if you want to - it's totally your choice.
Choose the one you like, either for a good reason or for no reason at all. If you simply can't decide, then print this post, and staple it to the wall. Throw a dart at it (or a limp spaghetti noodle will work equally well.) The grading scale closest to where you hit the paper? That's the grading scale option that you should use - it's the right one for you!