create table t1 (col1 varchar(10));

insert into t1 values ('96%');
insert into t1 values ('90%');
insert into t1 values ('100%');
insert into t1 values ('88%');
insert into t1 values ('34%');
insert into t1 values ('77%');

select col1,case when substr(col1,1,length(col1)-1)-40 >=0 then substr(col1,1,length(col1)-1)-40 else 0 end score
from t1

      col1     score
-------------------------
1    96%    56
2    90%    50
3    100%    60
4    88%    48
5    34%    0
6    77%    37