How far can you see on a perfectly clear day? For instance, do you think you are seeing up to 200 miles out, 300? Maybe less?
Well done.
Squink’s site might not take account of atmospheric refraction. My program gives me just under 245 miles. It is common in determining radar ranges to use an earth with a diameter of 4/3 of the actual diameter to account for refraction and that what the program I used does.
So take your pick since both answers are pretty close, differing by only about 5%.
A little further checking reveals that Squink’s cite does take into account refraction. It’s just that the formula I use apparently has a stronger refraction effect.
The geometric distance without regard to refraction is 212 miles.
So anywhere around 230 to 250 depending on the atmospheric refraction on your particular day ought to come close to covering the answer.
When you are about 30,000 feet in a plane
Maybe more?
93 million miles give or take. 
That first link owns. Thanks.