Finding value of resistor between two diodes
The voltage the analog pin of the Arduino will see is

$$ V_\text{analog pin} = \frac{ 3.3V-2 \cdot V_\text{diode drop} }{R_1+R_2} \cdot R2$$

The diode drop depends on the current throug the diodes, which depends on the value of R1.
When the current is in the milli-ampere range, you can assume a voltage drop of about 0.6V and 0.7V.