supplementing question

Sept07b2b

Ok.  This is a real supplementing question.  I am at a point where I pump about 8-10 oz in my 3 pumps while at work.  My LO eats 16 oz while I'm gone (she still won't eat real food).  I don't want to mix BM and formula just in case there is left over, I really don't want to waste the BM, so what I've been doing is sending 2 bottles of BM and 2 bottles of formula (each 4oz) and saving any left-over in case I have another drop in supply (my supply has been slowly dropping for about a month or so). 

Is it better to continue to save the extra?  or should I be sending all the BM each day?  Basically, should I try to continue to send half BM for longer? or more BM for a shorter amount of time? 

TIA!