Consider the circuit in picture, used to measure the voltage across diode and current flowing through it.
With this circuit I do measure the exact voltage across diode (\$V\$) with voltmeter, nevertheless the current measured with ammeter (\$I\$) is not the current passing through the diode, since \$I=I_d+I_v\$.
To evaluate the error committed doing the approximation \$I=I_d\$, is it correct to see if $$V/I << R_v$$ ?
Infact, supposing in first place that the approximation \$I=I_d\$ is correct, and also interpreting the ratio \$V/I_d=V/I\$ as the "equivalent resistance" of diode, then, using the current divider, if \$R_d <<R_v\$, then the approximation is justified a posteriori.
In other words is it correct to interpret \$V/I_d\$ as the resistance of diode, and consider the diode equivalent to a resistor for this porpouse (i.e. to evaluate this approximation)?