Unfortunately, I don't have a relative sense of how much resistance the flash tube is adding when the capacitor discharges, and it is not something that can be easily measured, because the resistance of the tube is going to change over time as the capacitor discharges. However, because of how much energy is being released as light/heat, I would guess that the resistance is pretty high. Perhaps the more experienced members on this site can give a more exact answer, but all I can recommend for now is that you try a resistor with a value of something like 1 mega-ohm (1,000,000 ohms), and then experiment from there. If the flash time seems to be the same length, try a lower resistance value, and if the flash does not go off at all, try a higher resistance. If you have any potentiometers, (variable resistors) those would work excellently for tuning the resistance to what you want.