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The 3:2 height to half base granite angle leads us to the fine structure constant and the theoretic 274.072 rc. Now the Titan relation leads to 274.16 rc. This along with a possible 274.69 rc relation explains the planned base slope. But the designers wanted to encode more so the angle was decreased toward the top. Your 51.2 degrees value is exactly correct and I believe this is the minimal angle. But as you say your 261.5 rc value has not been confirmed while present estimates of today's height are around 260.5 rc. Also according to your values:
(4 m/2) x tan(51.2 d)= 4.75 rc
Thus 261.5 rc + 4.75 rc = 266.25 rc
I don't know where you get 266.7 rc from. Now if you are correct in the 4 meter truncated top then the original height might be:
260.5 rc + 4.75 rc = 265.25 rc
But the first value I gave was 265.14 rc. Now let's the notice the correct name for Greece in Ionic and non Ionic alphabets respectively:
ELLAS = EKsAS(sextant) = 266 (height)
HELLAS = 274 (max angle height)
We could also consider the possibility of a capstone whether it was ever put in place or not. In this case the original truncated top would have a height of 1/a = 137.03599911 ancient meters or 449.73 feet.