Because of the lack of a circuit diagram and any information I can only give you an example. You will need to add the different values of resistance an inductance of your relay. You will also have to include the capacitance of the relay coil for better accuracy. But this will give you an approximate answer.
Example:
Using a 2N2222 NPN transistor connected up as a common emitter switch with the relay connected to the collector and +12 Volts supply. A 100 Ohm resistor connected in series with a MURS320 reversed biased diode and connected across the relay coil. The transistor is switched on for 100 ms every 500 ms and the input to the base resistor is a pulse with 5 Volts amplitude and a 10 us rise and fall time.
Coil resistance 10 Ohms
Coil inductance 1 mH
Q1 on resistance very small so will be ignored.
Rise and fall time 10us
Formula:
Coil current = 12 Volts / 10 Ohms = 1.2 Amps
Voltage across R1 = L*(1.2 Amps / 10 us) -12.75 Volts = 107.25 Volts
The waveform shows 110.15 Volts. That's not far off.
Fig1: Circuit Diagram
Fig2: Voltage waveform across R1