Duct work?

[amberteresa]

i am currently remodeling my bathroom on second floor. while in the process id like to run heat up to 3rd floor bedroom above bath. I have easy access to get it up, but have questions if heat will work efficiently. the current duct size is 4x10 or 40 square inches. i have been told by some that i need to use half of the square inches or 60 % . is this true will i still get good heat to both if i use half on each run?