supplementing with formula

sweetestlife28

thanks to advice from you guys, i started supplementing this weekend with gerber good start formula because i can't seem to pump as much as my baby needs lately. 

 as i am packing his bottles for the day now, how much extra do i need to include? he usually takes about 10 oz breast milk and lately ive been pumping about 5-6 oz.  so do i need to add 5 oz of formula to make it to the 10 oz, or do i need to add more than that?  

 thanks!