Need help building a complex(ish?) formula;

=(INDEX('Set Prices'!C$4:C$15,MATCH($B4,'Set Prices'!$B$4:$B$15,1)+1)-INDEX('Set Prices'!C$4:C$15,MATCH($B4,'Set Prices'!$B$4:$B$15,1)))/(INDEX('Set Prices'!$B4:$B15,MATCH($B4,'Set Prices'!$B$4:$B$15,1)+1)-INDEX('Set Prices'!$B4:$B15,MATCH($B4,'Set Prices'!$B$4:$B$15,1)))*($B4-INDEX('Set Prices'!B$4:B$15,MATCH($B4,'Set Prices'!$B$4:$B$15,1)))+INDEX('Set Prices'!C$4:C$15,MATCH($B4,'Set Prices'!$B$4:$B$15,1))