You will always have some amount of subtraction when two frequencies meet at equal level between 120º and 240º of phase offset. What you need to do is make sure that your speakers are within 2/3 wavelength of the highest frequency at which they could combine.
In our case, we are crossing over at 80Hz, but to be safe I used 90Hz for the calculation since the Sub and Main won’t be level isolated right at 80Hz. Then I found the wavelength of 90Hz, which is 12.5 feet. After that, all I needed was 2/3 of that, for 8.3 feet, which I rounded down to 8 feet.
Those steps again:
1. F = the critical operational range of your subs
2. max distance from center to center = (speed of sound / F) * (2/3)
Warning: the pattern will narrow as frequency rises. The tradeoff of pattern control down to 30Hz is that you may now be too narrow at 80Hz. The solution? Create a physical or delayed arc.