According to My research I have found that yes and no that you can and can’t base the line voltages off of the insulator number of discs.
Here is why you could NOT:
High voltages act differently in cold, hot, moist, or and dry areas. In Utah there are power lines with 12 ceramic discs on the power poles holding or suspending the power lines. The power lines only have 138 kV flowing through the power lines. But the number of insulating ceramic discs suggests that a little more than 161 kV is flowing through the power lines. So the answer for this is no, you couldn’t base voltages flowing through power lines on the number of ceramic discs on the power lines.
The reason why you could:
The main and only reason why you could base the voltage flowing through a power line off of the number of insulating discs is if you wanted to be over safe. Or what I call over-protective. Which means basically you are estimating higher than needed to make things even safer.