A good trick to planting is to place down those wood stake walls that you get early on.

Set them up in a straight line and then another 90 degree straight line to the left or right.

Use those stakes to measure your first row of barley/flax and then use the other one you placed to measure your columns of barley/flax. And once you have the measurements just work from there as you know how far apart to place them.

Make it even easier for your self to measure. Use some coal and wood. Place signs along the logs and give them a letter going one direction and a number going another. And use that to figure it out from there. And in doing this you will have built your self a reliable measuring tool. Afiak it is every 2 logs. So plant a flax/barley skip 2 logs and plant another one.