I’m not sure how accurate this is but there is definitely a strong negative correlation between shield strength and the threshold value:
effective shield capacity = max_capacity * (1 - offline_threshold)
Shield will come back when both of these are true:
time offline >= max_capacity * offline_threshold / regeneration_rate
time offline >= offline_rebuild_time
To ensure any missiles you add don’t blow up too early here are some simple formulas for calculating the minimum time they need to reach their desired distance:
top speed = muzzle_velocity+accel * motor life
accel distance = muzzle_velocity * motor life + accel/2 * motor life
munition lifetime >= (seeker_range - accel distance) / top speed + motor life