How many amps of Current is consumed in this circuit?
You simply have to work with the power dissipation for input and output to derive the input current.

The formula:

Pout * 1/efficiency = Pin 

This becomes:   

(Vout * Iout * (1/efficiency))/ Vin = Iin

For example, if the R'Pi draws 200 mA @ 5.2 V and the input voltage to the Powerboost 500 is 3 V.

(5.2 * .2 * 1.1)/3 = 380 mA

IF Vin were to fall to the minimum 1.8 V

(5.2 * .2 * 1.1 )/1.8 = 635 mA  

Note: The Powerboost 500 is really configured to support a single Li-Ion cell, so the input low indicator comes on at about 3.2 V and less, so if you operate this from a 3 V supply the indicator will be always on.

I have powered a Raspberry Pi with this solution and it works well. I have tested it out to over 800 mA @ 5.2 V with no problems, though I use a single Li-Ion cell.