The fillet is to be the lesser of the plate thicknesses being joint but not less than the table. Your plates are 12 and 10mm. The lesser of those two is 10mm. Checking the table reveals a lower limit of 6. 10 is not lower than 6 so so you use 10. To clarify - if your plates were 12 and 5mm, the lesser of the plate thicknesses is 5 but the lower limit from the table is 6 so your weld would be 6. In other terms it is the greater of ( table and lesser of ( plates being joined ) ) Hope that helps...