Paul,
I would say there are a couple of things you should keep in mind.
First, know that there is likely never going to be a "perfect" time delay setting that would remain constant through out the entire listening area. For example, if your main PA is flown (which is a good idea in many situations) the distance between speaker and listener's ear will vary from the front of your house to the back of your house. (obviously this will happen even if it's not flown as well, but more on that in a second) So say you pick a spot in the middle of the audience area, measure your distance to the backline (a laser tape measure is a great tool for this by the way), then measure your distance to the mains. Do some basic subtraction and you'll have the difference in distance between the two systems and a good place to start for determining your alignment delay settings. But now you move to the front row and of course your numbers will change. Move to the back row and they will change again. So no matter what delay setting you use, it won't be perfect for every seat in the house. The bigger the venue, the more difference there will be. Of course at some point the venue will get so large that the stage sound will hardly matter or matter a lot less at least. So what do you do here? Typically I would take measurements in the front of the audience area, rear of the audience area and in the middle. And then pick an average that seems to work best. If your mains are flown then the distance difference between the front row and back row will be less (which is why flown PA works better than ground stacks... in very general terms). Of course if the PA is ground stacked, then the difference in distance between back line and PA will be more consistent. The only way it would be perfectly uniform would be if they are at exactly the same height in relation to the entire audience. (that would also mean the audience is also all on a single plane... or a flat floor... no balconies or rakes to the floor or upper levels, etc....) I'm sure somewhere there is a venue like that, but I think it's pretty rare. So any variation in audience height in comparison to the backline or the PA will mean that there is not going to be a "perfect" delay time across your entire audience area.
Second, you mention that the venue is small enough that you don't even need overheads? If so, how much of the backline are you even pumping into the main PA? Now I don't mix club acts very often so I am probably out of touch with what is typical these days. But the reason I ask is if you're not amplifying the backline in the PA, then really I would say it might be a waste of time. The reasons (as I understand them) that some engineers will align the PA to the backline is help prevent some comb filtering from happening. Again it won't be eliminated, but it could help clean things up, particularly in lower frequencies to have the bass amp's sound arrive at the same time as the PA, just as one example. Anyway if most of what the audience hears from your backline comes from the backline itself, it might not make all the much difference. And if you don't amplify the backline at all... I definitely wouldn't bother. That's my thoughts anyway. Typically when I am using delay in the mains to align to a live sound on stage it is to vocalists where a reasonable portion of what the audience will hear is the natural voice or it's acoustic instruments. But then that is also the nature of the type of work I am usually involved with, doing theatre rather than music acts or bands most days. In other words, I could well be wrong and my understanding of why some engineers time align to their backline could be off.
I would also second the advice you got on the SAC forum to use an analyzer if you really want to be precise. That will certainly give you the most accurate picture of what is going on. Room EQ Wizard is a freebie and can do those kind of measurements. Although you would have to do some of the work on your own since you would have to put out a test mic, take a measurement on the mains, then measure the backline and do the subtraction on your own. Other more sophisticated tools can do that last step for you. Either way I would suggest if you go that route, measure from multiple places in the venue. I bet you'll see that the "perfect" delay amount will vary across the room. So use an average of your measurements.
Or just break out a laser tape measure, do some basic math on a bar napkin and put that measurement in feet into your delay unit. I time aligned many systems over the years with nothing but a big tape measure, some paper and a scale rule. Oh... and I used the measuring devices that are permanently attached to my head as well. Probably the best tools you can make use of. Most days it will be close enough that it won't matter.
But it is always nice to measure with better tools and see how close you got the old way.