OK forget the percentage we'll go back to Linear regression.
With E1:-70 F1:-100 E2:- 50 F2:-70
putting this formula in C1 will return B1 adjusted by linear regression.

=SLOPE($E$1:$F$1,$E$2:$F$2)*B1+INTERCEPT($E$1:$F$1 ,$E$2:$F$2)

HTH
Martin