I believe the answer can only be empirical, not definitive.
To examine some of the figures mentioned:
there is a 20% inefficiency (which I do not know if it is true for most portable charger)
A portable charger that is itself charged from USB (5 Volts) would need a boost converter to be able to supply 5 volts at its output. Boost converters commonly mention efficiency of 65 to 85%. TI's TPS61030, TPS61031 and TPS61032 state 96%, and Maxim's MAX8815A states 97% efficiency.
These figures do not account for possible efficiency loss due to external components (ESR of capacitors for instance) or temperature variation. Thus, treat that "20%" number as indicative at best.
your phone needs power for stand-by, so in my experience, you'll have just 65% capacity.
That would depend on whether the phone is kept powered on while charging, what power intensive tasks (e.g. WiFi, social media polling software) are running on the phone, and even the current draw of the phone in the nominal "powered off" state - Some smartphones do not actually power off completely unless the battery is pulled out.
Thus, that 65% number is also indicative at best, though varying it somewhat is within the user's control.
by industrial standards for batteries is +/- 20% tolerance admitted with capacity.
That number would be defined in the datasheet of the specific battery in question. It would also vary widely by age / charge cycle history of the battery, temperature, contact oxidation and possibly several other factors.
So, while the number is a reasonable guesstimate, it is not definitive.
Note that this last figure is applicable to both, the cellphone battery and the portable charger battery.
So, can one use the magical value 45% as a gauge for portable battery charger?
Clearly not. The only numbers that can be used, even as a rule of thumb, are those empirically measured for your particular situation and use pattern. Even so, the percentage will change widely over charge cycles, season and time of day (temperature factors).
You probably blew out the fuse on your ammeter. An ideal ammeter has zero resistance, so when you measure current you need a load.
Poorly made picture:
Notice the light-bulb providing a load. Try connecting a 100Ω resistor in series when you test, if you're getting 5V out you should get 50 mA. You can try lower resistances, but you'll want to ensure that the resistor you use has a high enough power rating. If you use a 100Ω I'd go with a 1/2 W resistor to be safe. If you only have 1/4 watts lying around then try a larger value resistor.
Best Answer
Those numbers tell you the current that the supply can provide, not how much your phone needs. Your phone will only draw as much as it's designed for, and so you don't know how much power is coming from the mains. So, they are correct. You can't get anything meaningful from the adaptor text itself.
There are two ways to get this figure. You can measure the current going into the adaptor itself, or you can measure the current going to your phone. The latter means you also need to know the efficiency of the adaptor as there is some loss in the adaptor when it converts mains voltage to 5 V. You could potentially estimate it from the mains current input (0.3 A), but that doesn't say what voltage or frequency it's at.
Adding calculations for the second method Ok, so your phone is probably charged over USB which used to be limited to 500 mA. Also, that 1.55 A is the maximum the supply can provide, you don't want to have that all the time. So, let's assume (crudely) that \$ I = 500\ \mathrm{mA} \$. Power to your phone is \$P_{out} = IV = 0.5 \times 5 = 2.5\ W\$. Then, let's assume the adaptor is 80% efficient, so \$P_{in} = \frac{P_{out}}{0.80} = 3\ W\$. The multiply by the time to completely charge, and you're there.