Is there any standard (regarding height/distance) for middle eyelet on
No, there is no standard. Many bikes don't have a middle eyelet on the fork.
Some bikes don't even have eyelets on the dropouts.
So maybe there is standard for it, and if not, what would be safe
range (measuring from the crown of the fork)?
People usually buy a bike and then find a rack that will fit the bike.
Even if there are no eyelets there are still ways to get a rack onto a bike using clamps.
Rack makers usually build a rack with enough flexibility to fit a wide range of bikes. Even then some creativity is often needed to get things to work.
Here is an example - Nitto M18:
This front rack is made for a bike with middle eyelets. There is also a version that comes with clamps to accommodate bikes with no middle eyelets.
It comes with hardware that allows it some flexibility on how it attaches to the crown of the fork. The thin metal bracket can be bent and adjusted.
The stays that connect to the fork can be adjusted longer or shorter.
It's possible to find a rack that can be made to go on any bike because there are a wide range of racks designed to solve a wide range of problems.
Here's a rack installation with some creativity.
Water hose clamps and some interesting spacing on the mounting bolts.
A different rack would have offered a more elegant installation solution. This is a case of working with what you have.