I use a kitchen scale.
I get the area of the plot, figure out what I want in it, and then figure how much I need of each type of seed. I weigh out each component and dump it in a bag and label the bag. I give it a good shake to mix it up before spreading and dump it in an Earhway chest spreader and do a ring around the perimeter of the plot and then a criss-cross grid until I run out.
I used to mix up a batch by acre, but I quit doing that because I end up making changes to the plan before the next planting, and each plot is different.