P-N Junction current calculations
This is actually rather simple, given your assumption. We can apply kirchoff's voltage law, which states that the sum of the voltages around a loop equals zero.
$$V_{in} - V_R - V_D = 0$$
With \$V_{R}\$ The voltage over the resistor, \$V_{D}\$ voltage over the diode and \$V_{in}\$ the voltage you apply to the circuit. Solving for \$V_{R}\$:
$$V_R = V_{in} - V_D = V_{in} - 0.7$$
Simply fill this in in Ohm's law:
$$I = V_R / R = (V_{in} - 0.7)/R$$
Edit : This formula is only valid when \$V_{in} &gt; 0.7V\$ as Warren said in the comments. A voltage lower than that would not induce any negative current. (Assuming an ideal diode)