Back to work - pumping question

Al
Hi all! So I'm going back to work MWF on Jan 16th. Ive worked with my lactation consultant to come up w a good feeding & pumping schedule, & have started practicing some. 
In practicing, I'm pretty sure baby will need 4 4-oz bottles & I will be able to pump 3 20-min sessions. In 20 minutes I can get 3-4 oz. Hopefully as I improve, will consistently get 4. So, I will probably be able to pump 12 of the 16 he needs. I will get the additional milk by pumping between feedings when I'm home. My question is -
How much will my milk supply be impacted by only pumping 75% of what he needs on those days? If I let him eat as much as he wants the other 4 days, will we be ok? I keep reading "you need to pump what baby eats while away from him" but I'm just not yet pumping that efficiently! Is everyone else able to pump enough at work,or having to supplement w additional pumping sessions on home days?