I use an alternative I got on this forum years ago that completely ignores the numbers multiplier. Still works reasonably well. I may try the method(s) mentioned above, but I just plug in numbers and I'm good to go.
But the main thing? No calculation will ever be 100% accurate. There will always be variation from group to group, along with types of creatures, how many encounters the group has had prior, environmental factors. Just as important? How effective is the group at combat and, of course, how effectively the DM runs the monsters. I've run for two different groups of the same level with the same options at the same time and one group was simply able to handle far more than the other.
I've adjusted my expectations for my group based on experience and it works pretty much as expected at least 80% of the time. Sometimes the dice are hot, sometimes I set up the environment to be a little too advantageous to the enemy or optimal tactics would simply be boring. So nothing is perfect, but you can get pretty close.