The physical path length is far from the only factor involved in correcting the time alignment. Crossover frequencies, filter types and filter slopes along with the phase response of the other boxes in the system will have a major effect on the time alignment. One of my customers found this free software which will allow you to do an impulse response measurement. Using the impulse response you can align the various components of your system.
Using the impulse response measurements you can make adjustments in the crossover filters and delay times to achieve the best possible alignment. To suggest a delay time based on the path length of the horn without considering the crossover frequencies and slopes would be essentially misleading. Further, if you’re using certain kinds of amplifiers the net time alignment could be further offset by up to 2ms. There is a super-simple method for aligning speakers that doesn’t involve analyzing graphs. It’s not absolute but it works a lot better than guessing. And it’s also free.
Put the speakers next to each other facing the same way with the sound sources as close to each other as possible. Using a tone, which can be sourced from an app for a phone or a track downloaded from a website, drive both speakers to the same level. (HIgher level makes it easier but don’t blow your speakers!) If both speakers operate in the same passband, the frequency of the tone should be near or below the center frequency of the operating band .(If your subs run from 30 to 100 Hz then you would use a tone of 45 Hz or 50 Hz for example.) If the two speakers operate in different pass-bands then the tone used should be the crossover frequency.
Hold a thin piece of paper, such as normal printer paper, between the two sound sources and adjust the delay until the paper doesn’t move at all. (You may also need to polarity invert one of the boxes to get a better alignment.) If the pressure on one side of the paper matches the pressure on the other side of the paper then the sound sources are aligned in time. It’s an analog concept in a digital world…
Once you have them aligned in that position, if you then move one speaker backwards relative to the other you can measure that distance and apply delay of that distance to the forward-most speaker.
This should allow you to align practically any two woofer-type boxes together, if you have a method for delaying them….
If you need any help please let me know all the components of your system and all the settings you’re currently using and I’ll see if I can make any suggestions.
Have fun and let me know how it works out…