My probe is about six inches from the fan, there is a high turnover rate so with probe and fan at the bottom blowing across (in my setup) it works great. I have seen people recommend against putting the probe in the egg box (Possibility of driving the heat too hard and overheating), but there are also people who put it in the egg box and still have good results. If there isn't time for you to test different methods then you will have to go with whichever one is giving you the most reliable results.
I would agree with Jerry when he said to put the Hydrofarm on your rack to free up a Herpstat for your incubator, you would be happy that you did in the long run.
I noticed that you have the heat attached to the bottom of the incubator, this might make it harder to get the uniform temp throughout. Anything on the bottom shelf will be right over the heat and it may not distribute as evenly to the top of the incubator. Most of the designs I see have the heat running up and down the side/s or at the back. Generally this would allow the heat to be more evenly distributed by the circulation fan.
Hope this helps.