Dear Massarrat,
The formula as it is now will unfortunately calculate each month the same way by adding previous month YTD amounts to current month MTD amounts, including for the month of January, which is incorrect of course.
In order to have a different behaviour for January (where YTD=MTD), you need to add an IF condition to prevent the addition of previous month YTD amounts in January, as follows :
{AC in (CLASS=P) ; FL = Y99} = IF ([DP<>+0.01] ; {DP =+0-01 ; PE = +0-01}) + {FL = M99}
Regards,
Olivier