uhm...
i think that you can first fix your slew rate..
you should have a slew rate of 1V/ns or greater, equal to 1 GV/ s. So, the current which the voltage-to-current converter has to source when a large signal applied is simply I=C*dV/dT = 50e-15*1e9 = 50e-6 = 50 uA. This is the maximum output current you have to set for your OTA. This current can be higher if you need a greater slew rate. 50 uA is the "minimum maximum" current.
Then you have to measure your settling time by applying a small signal, to avoid reaching the slew rate, and evaluate the settling time itself, ringing and correct it if necessary. However, i think that the settilng time spec is for a given voltage, isn't it? This is because if you apply a signal which makes the OTA to reach the slewing, the settling time is superimposed by the slew rate itself, because the OTA works at its max speed.