how many feeds will you be away from bub for? if its only one you might not have to supliment it at all with formula.
i went back to working 2 jobs from when my daughter was about 6 months i expressed most but then after a couple of months i couldnt get enought out. so i spoke to the ABA and the suggested that she just have her food(was on solids by then) and water. until i get home to feed her.
this worked great for us

HTH