>Total/obtain*100
>Total =tobt and obtain is =CAST(SUM(IIF(result="Fail",1,0)) AS I)
>

>TRANSFORM(ROUND(count(tobt)/CAST(SUM(IIF(result="Fail",1,0))*100,2),"##.##") AS PERCENTAGEu;

>i am missing something

TRANSFORM(ROUND(;
                COUNT(tobt);
                  /;
                CAST(SUM(IIF(result="Fail",1,0)) AS I) *100;
             ,2),"##.##") AS PERCENTAGEu;

you can omit casting here:

TRANSFORM(ROUND(;
                COUNT(tobt);
                  /;
                SUM(IIF(result="Fail",1,0))*100;
             ,2),"##.##") AS PERCENTAGEu;

More correct is:

TRANSFORM(100*ROUND(;
                COUNT(tobt);
                  / SUM(IIF(result="Fail",1,0));
             ,4),"##.##") AS PERCENTAGEu;