Yeah having that much open road can be tough in many areas. I did mine on the trainer in college, which adds another element of suffering of course because it is so boring. Fortunately for me, I have Fiesta Island in San Diego now, which is an essentially infinite ~4mi loop with no intersections . I do much of my weekday riding (intervals) there as it is safe, open and only 20min of easy pedaling from home.

I don't mean to hate on your max HR methods, I think it's cool that you're interested in the first place, and it should get you in the ballpark. The LT method is just more functional and accurate as all, no biggie. It's also kind of cool that the disparity between the formula and what you obtain in field tests is fairly small - perhaps the formula is good for you.

Do you have a protocol for determining max HR? I would probably do something like ride pretty hard for a couple minutes, ride real hard for 60s, then sprint all out for another 15-20s. I've never tried to formally test max HR tho, so IDK. That also sounds like a good way to puke too, if you're trying to lose weight haha