When to use "Made in the USA"

So the product I make is a kit to start off. There are off the shelf parts (small motor/bolts/bearings/t-slot) made in china/indonesia/japan. And there are the custom in house parts I make right here in the USA that really make it into the machine that it is albeit idk what % it is (by parts/weight/importance?).

I have read through the ftc guide lines but it seams very lax on specifics.

Any thoughts would be helpful!