Estimate minimum current draw for a device
The minimum will highly depend on the ic and what it is doing. Some microcontrollers can go into a deep sleep state and draw microamps, while their idle but not sleep state could be milliamps.

You could use the datasheet where quiescent current is listed (Iq), or if you have a specific board in mind, build one and measure the current draw while idle or asleep, using that as your baseline. 

Your adc lists this as power dissipation. At full activity, 78 mA at 3V or 236mW. Then it shows shutdown and nap modes at 2 and 15 mW, essentially 0.65 mA and 5 mA. Leakage current may bring this up a bit, so 1 and 6 mA. But that's only if you use the sleep or shutdown modes. 

You best case is taking advantage of sleep modes for all parts of your board, and measure to confirm. Obviously you want a quality ammeter.

The 10% is a rule of thumb. It won't beat actual measurements, but at these current draws 10% should be good enough. There are diminishing returns in lowering these tolerences.