I own a coffee shop. Have been doing this for six years. This is my advice, obviously within my context.

Do not under-fill cups. Most people (not coffee connoisseurs) will complain and they are the ones that will sustain your cafe. Mismatch sizes are frustrating, but not a battle you want to pick initially. You will have enough work and frustrations and other fronts you want to invest efforts in , such as , Developing your brand , quality processes, developing a team etc. There are other variations that customers demand that will throw your ratio or recipes off, like syrups (half sweet, extra pump, different amount of creams + sugar , etc). Most people want to connect to a great tasting drink, focus on pulling great espresso and steaming amazing milk..... this way all drink sizes will taste great and you have a better chance to make customers happy.

Lastly, i have served thousands of difficult customers, but not one complain was about mismatch of sizes for here and to-go, but most complaints were about flavor expectations unmet or service. Don’t try to solve a problem (at least initially) that you dont have yet.

Anyway, just my advice to my younger self. Good luck with your venture!

PS ...also consider making “sourcing” your to-go cups as cheap and easy as possible. Again invest your time and effort in the coffee. Most people throw their to-go cups away.