The overall efficiency from a power bank to a smartphone battery is about 2/3, according to this source:
The amount of energy a battery can provide depends on its voltage (nominal 5V for a USB power bank), the size of the supplied current in Amperes and how long the current can be kept flowing. Since the voltage is fixed (5V), current and time are sufficient to express the capacity of a power bank. The units of current and time are then used together: 10,000 mAh means that a current of 10,000 mA (milli Ampere, so actually 10 A) could be provided for 1 h, or 5 A for 2 h, or, since common charging currents for mobile phones are around 2 A, this current could be provided for 5 h.
To get 10,000 mAh on the smartphone side, about 15,000 mAh will be drawn from the power bank. In other words, the required margin is about 50% of the desired capacity.