If you don't mind using a helper column...
In C1 (copy down)
in D1 (copy down![]()
=COUNT($B$1:B1)
![]()
=IF(COUNT($B$1:B1)<10,"",IF(B1="","",(5+(SUM(OFFSET($B$1,MATCH(COUNT($B$1:B1)-9,$C$1:$C1,0)-1,0,ROW()-(MATCH(COUNT($B$1:B1)-9,$C$1:C1,0)-1),1)))/2)/10))
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