这样吗?
IIF([品种]= '铁盘' And [承载] >= 0.5 And [承载] <= 2.5,3/4,IIF([品种]= '铁盘' And [承载] >= 2.6 And [承载] <= 6,4/5,IIF([品种]= '铁盘' And [承载] > 6.1,5/6,0)))