Flame probe for uniform field

I used this equipment to measure the potential in the uniform field between two parallel plates. I had a potential difference of 600V. I measured the angles of the gold leaf at 0 mm from the negative plate and then when it was touching the positive plate. I was then able to calibrate a scale to turn other angle measurements into voltages. I made this diagram to explain how it works, to amplify the explanation I gave in my previous post:

The closer the probe tip is to the positive plate, the more electrons are attracted from the gold leaf and the bigger the angle. Work is done against an attractive force to pull the electrons up so the angle represents potential energy. The graph looked like A below:

If we pulled the plates apart, the potential difference would be the same but the field would be weaker. So field strength is represented by the gradient of this graph. However, we went from - to + whereas the direction of an electric field is from + to -. To sort this out, we insert a - sign:

Field strength = - potential gradient