Index on padl(alltr(STR(SkNo)),4,'0') Tag SkNoA

>How might one go about this?
>The field is: SkNo N(4,0)
>
>Index on SkNo Tag SkNo
>Index on STR(SkNo,4,0) Tag SkNoA
>
>Data:
> 250
> 350
> 680
>6500
>
>Letting the user type, seek on the tag SkNo requires the full number to be typed; seek on the tag SkNoA brings you to 6500 before 680. What index could be created to get to 680 first and 6500 second?
>
>Thanks,