To calculate the rank for a value in A1:
=INT(A1/5000)+1+(A1>=2500)
If the above formula is in B1, then the next increment is found by:
=(B1-1)*5000+(B1=1)*2500
and the % shortfall from the next increment is thus given by:
=1-A1/((B1-1)*5000+(B1=1)*2500)
whilst the % increase required to reach the next increment is thus given by:
=((B1-1)*5000+(B1=1)*2500)/A1-1
(do note that the last formula will fail if the value in A1 is 0).

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]