Sizing a Commercial Water Make-Up Line

Johnny6

I am working on a commercial project with a 2 million BTU boiler and was wondering if there is a rule of thumb for sizing make-up water lines. 

I mostly work with residential and small commercial projects and they usually only requires a 1/2" line to bring in water. That number is just from memory mostly and to be honest I have no way to justify the line size. 

 s there a way to calulate this line size or a rule of thumb based on some requirement for the boiler?

Thanks,

           John