How do I determine the proper ratio of CO2 and N for my tank?

[Jlat]

I wish to fill a 60 cf tank with both CO2 and N at a 25% - 75% ratio. I have been told by my supplier that if I can tell them the "Recipe" for my size tank that they could make this happen for me.
What I need to know is the amount of liquid CO2 (measured in pounds) that would be placed into the tank first and then the amount of N that would then be put 'on top' to reach my desired blend.

Thanks,

 

[BvU]

Hello Jl,

Would that be a mass ratio or a mole ratio ?
What's in the tank initially ? How do you get it out ?