So, in that case, they'd have to be level 18 (18 x 1.5 = 27). But that formula doesn't take into account the number of party members. 2 single level 18 characters would get slaughtered by the Great Wyrm, while 8 level 18 characters would mop the floor with it (even though both groups would have an APL of 18).
You have to mix the two together. Frankly, no single monster stands up well to a full group of high level characters. They always need some sort of backup.
So the Lazy Encounter Benchmark gives you both –
four 14th level characters gives you a LBM of 28 with a single creature being potentially deadly at 21. If you want to edge closer to that benchmark, you have to throw in some side monsters. Maybe a couple of fire elementals.
I also have a dial on the benchmark you can turn if your 14th level characters are pretty powerful: Increase the threshold to 3/4 of character level at 11 to 16 and 1:1 at 17+.
Remember, the benchmark isn't a hard rule. It's a loose gauge to give you a vague idea how powerful an encounter might be.
Step 1 is to build encounters that make sense given the situation in the world.