Here is my first planting grid built using PVC pipes and nylon thread (total cost $5). I call it "SPACE MASTER".

I have spaced lines 4" apart, that adds 36 squires in 2 sqft. I can use this grid to plant at various spacings;

1. very large plants @ 24" = 1 plant (tomato)

2. large plants @ 12" = 4 plants (cabbage)

3. medium plants @ 8" = 18 plants (bush beans)

4. small plants @ 4" = 36 plants (onions)

5. very small @ 2" = 144 plants (carrots)

Cheers -Chandra