Estimate the speed of BLDC motor using back-EMF constant
After some research i found the solution.  thus, i will answer my own question
Some motor spec use \$ke= Vrms/rpm \$(because when calculating the Ke they used the VRMS volatge and not the peak voltage) so in my example the \$Vrms=40*0.8/sqrt(2)\$
but most motor manufactures use \$Ke= Vpeak/rpm\$ so in my example \$Vpeak=40*0.8\$ 
and in order to calculate the rpm i need to take into consideration the voltage drop and the losses therefore the actual speed will be \$rpm=Dutycyle*(Vsupply-Vdrop)/Ke -(Ploss/Tloss)\$
with \$(Ploss/Tloss)\$ is the speed reduction brought about by the power and subsequent torque required to be produced by a motor to overcome friction, windage and core losses.
source:https://things-in-motion.blogspot.com/2019/05/understanding-bldc-pmsm-electric-motors.html