This really comes down to personal preference. Allow us to explain. 

Regardless if you are at your favourite café or at home with your Nespresso® capsule machine, and regardless of whether you’re drinking espresso, long black, flat white or cappuccino etc,  your coffee is based on a 40ml shot of espresso. 

In a café that 40ml is extracted from approximately 8.5 grams of coffee grinds, however Nespresso® compatible capsules only use approximately 5 grams of grinds.  

Therefore a café can serve a 200-250ml flat white without compromising the strength. Whereas a capsule based coffee should really be between 120-150ml to achieve the same strength. 

However everyone’s preferences are slightly different. So play around with the volumes of your milk or water to find your preferred strength for your preferred blend.