Belle - I find my boobies are engourged in the morning...so i always express around 20mls from whichever boobie I am offering...then I empty that boobie after Paige has finished her feed....that way not only does the boobie replenish itself for later on I am also banking enough milk to offer and EBM feed in the afternoon.
They do say that some boobies run low in the afternoons...so by offering more boobie in the arvo, it may help to have bubs full and content for their evening sleeps (that's what the ABA councellor told me anyway).