=SIERREUR(SI(C8<>"";C8;((MIN(C9:$C$90)-MAX($C7:C$8))/(INDEX($A:$A;EQUIV(MIN(C9:$C$90);$C:$C;0))-INDEX($A:$A;EQUIV(MAX($C7:C$8);$C:$C;0))))*A8+MAX($C7:C$8)-((MIN(C9:$C$90)-MAX($C7:C$8))/(INDEX($A:$A;EQUIV(MIN(C9:$C$90);$C:$C;0))-INDEX($A:$A;EQUIV(MAX($C7:C$8);$C:$C;0))))*INDEX($A:$A;EQUIV(MAX($C7:C$8);$C:$C;0)));"")