I'm planning on setting up a low-HF antenna for near to mid-range contacts.
In my particular case, ideally I would like to be able to cover a radius of approximately 1000-1200 km with good reliability, operating on 3.5 MHz and possibly supplanted by 7 MHz. (Using a combination of the two bands to achieve full coverage is perfectly acceptable.) At a minimum, I want to be able to cover something like a 600-700 km radius. I'm at approximately 58°N, locator JO68; this of course means that I need to take into consideration large seasonal variations in ionosphere illumination.
Now, such coverage seems to me to be a nearly ideal fit for NVIS.
Given an antenna height above ground and operating frequency, how do I determine an approximate resultant coverage radius in the case of NVIS?
Or alternatively, given an operating frequency and desired coverage radius, how do I determine how high above ground the antenna needs to be?
I'm preferably looking for a (mathematical) answer that does not involve using antenna modeling software.
For simplicity's sake, let's just assume a straight half-wave dipole at an even height above perfect ground.