Hello, Try this,

=IFERROR(SUMIF(A12:A33,"<="&A36,C12:C33)+(INDEX(C1 2:C33,MATCH(A36,A12:A33,1)+1)/(INDEX(A12:A33,MATCH(A36,A12:A33,1)+1)-LOOKUP(A36,A12:A33)))*(A36-LOOKUP(A36,A12:A33)),SUM(C12:C33))