Is there a way to estimate the actual temperature where the yeast ferments without having a sensor inside the carboy?

I was reading an article on fermentation temperature control where it says,

However, since yeast growth and fermentations are exothermic and therefore generate heat, figure that the temperature within the fermenter can be as much as 8 °F (4 ºC) higher than outside of the fermenter during the early days of fermentation. So beers that are fermenting in refrigerators set at 65 °F (18 ºC) are most likely fermenting at about 72 °F (22 ºC).

4 ºC is a huge difference!

Right now I have a carboy in a fridge that's temperature controlled to 19 ºC (on the lower side of the optimal range for WLP351). The sensor strip is taped to the glass of the carboy, and consistently reading 19.2 ºC.

I'm wondering if next time I should set the temperature control lower in the first 12 hours, but I realize I have no idea how that translates to change in different sections within the carboy—near the glass, the bottom, the top (where ale yeasts float, correct?), and so on. For example even if it's 19.2 ºC on the outside of the glass, the liquid touching the inside of the glass is probably much warmer due to convection.

Is there a gradient diagram or some rough formula that would help me determine the effective fermentation temperature (where the yeast actually is) from the outside glass temperature? Or, do I have to find a way to stick a (sanitized) sensor strip into the carboy.