Aladin, i tested your solution and it does go blank, but it doesnt give me the desired result as my original formula:
I have taken out the none of these for 5-9, because it will never occur in my records
=IF(OR(C3=0,C3=5),1,IF(OR(C3=1,C3=6),2,IF(OR(C3=2,C3=7),3,IF(OR(C3=3,C3=8),4,IF(OR(C3=4,C3=9),5))))))
Also, I tried added isblank in this way, but it gives me an () error:
=IF(ISBLANK,(IF(OR(C3=0,C3=5),1,IF(OR(C3=1,C3=6),2,IF(OR(C3=2,C3=7),3,IF(OR(C3=3,C3=8),4,IF(OR(C3=4,C3=9),5))))))
however, Fawlty1's solution gave me the desired result.