Value of resistance R from the equation: \$ R = \frac{t}{C\, \ln\frac{U_0}{U}} \$ for specific value of \$U_0\$ = 300 V at time t =0
Suppose you have a RC element, then for charging phase:
$$U=U_0(1-e^\frac{-t}{RC})$$
$$R= -\dfrac{t}{C \cdot ln(1-\dfrac{U}{U_0})}$$
For discharging phase:
$$U=U_0 \cdot e^\frac{-t}{RC}$$
$$R= -\dfrac{t}{C \cdot ln\dfrac{U}{U_0}} = \dfrac{t}{C \cdot ln\dfrac{U_0}{U}}$$



simulate this circuit &ndash; Schematic created using CircuitLab
At the time t=0 it can be solvable, since the discharging didn't take place yet. You have no valid measurement on base you could solve something. However the R could be anything except 0.
EDIT:
I think it's better to calculate the R for each time difference and only then use the mean value of R:
$$U_{n+1}=U_n \cdot e^\frac{-(t_{n+1} - t_n)}{RC}$$
$$R_n=\dfrac{(t_{n+1} - t_n)}{C \cdot ln\dfrac{U_n}{U_{n+1}}}$$