Hi Faisal,
As long as the isotopic composition of your sample matches the reference material(s), you should interpret (for example) V51_ppm as "the V concentration, calculated from the V51 channel" and it will be the total concentration.
Similarly, you can input the total Ca concentration for the Int Std.
The way I think of it is: if the isotope you're using makes up say 50% of the total, your signal will be 50% of the total signal of that element. But it will also be 50% of the RM signal for that element, and so it will cancel out in the equations.
I hope that helps? If you have any questions, please feel free to post!
Kind regards,
Bence