calculation – What is the probability that 3 randomly selected points in a circle form an acute triangle

Please note that it is inside, not on the circumference!
Glen Hall has addressed this issue in the literature, but I can not find any papers.
It seems that the answer is $$ frac {4} { pi ^ 2} – frac {1} {8} $$ but with my calculations, it gives something different $$ frac {44-4 * pi} {45 * pi} $$! I'm definitely wrong!
Help would be welcome!