I saw a tool where someone took a flat piece of wood (let's say 1x6 or 1x12) and put 1" dowel rod pieces every 3" or 6" (with wood glue, I guess...or maybe a very small nail). The other side might have similar pieces, but at another distance (let's say one side is every 3", the other side is every 4")

Using this piece of wood that is 6 or 12 feet, they would lay it down on the ground to be seeded, depending on how far apart they wanted the holes. Using either side, they now have holes that are every 3, 6, 9, 12" apart or every 4,8,12" apart, depending on what they need to plant. And of course, if you need something every 6" or 8" apart, then just skip every other hole, for example.

The wood will also help keep the rows pretty straight.

You can use any distance you want based on what you know you'll be planting.