On the vertical line you can choose the number of rows per 1.5m bed (wheel centre to wheel centre). Then choose, on the horizontal line, the number of onion sets that you wish to plant per linear metre of row. At the point where both lines cross you can read the number of onion sets to plant.
Example: if you want to plant 20 sets/m1 and 5 rows on a 1,5m bed, you have to plant 666.667 sets/ha.
This table can help calculate the accompanying amount of kilos to be planted for different combinations of set size and sets per hectare.
Amount of kilos to be planted in relation to set size, number of sets per kg and number of sets per hectare
Example: the above example shows that you have to plant 666.667 onion sets. In the 10/21 mm bracket you will need between 1.875 and 2.188 kg onion sets (2.087 kg to be precise).
The number of onion sets per kg can deviate 5% either way.