For sure, you'll have a null either way. The closer, the higher frequency the null will be. I would get get a mic and measure it in different positions with and without treatments. You may not want to be treating the point of first reflection per se. You may be better off attempting to mitigate the sidewall null. Depending on the frequency, you may need to treat a wider sidewall area to reduce the null. Also, it might be easier to deal with the null at higher frequencies. However, the null amplitude will likely be lower, the closer you get to sidewall. You also may not "hear" the null in exactly the same way it measures. It will take some work on your part to figure out what sounds best and gives the best results. IMO, you will save yourself much time and pain if you work through this with a mic.
Michael.