Application and calculation of the 4% presumed markup on manufacturer sales

We are a truck body manufacturer that sells truck bodies directly to end users as well as dealers for resale. For our dealer sales, we are sometimes the taxpayer for Section 4051 tax (e.g., we do not obtain a resale certificate from the dealer), but other times, we are not the taxpayer (e.g., we do obtain a resale certificate from the dealer). How does presumed markup apply to these sales, and how should it be calculated?