The phrase "a milestone in which" is correct and usable in written English.
It can be used to describe a significant event or achievement that serves as a reference point in a process or timeline.
Example: "The launch of the new product was a milestone in which the company established itself as a leader in the industry."
Alternatives: "a milestone that marks" or "a milestone representing".
But other, cheaper approaches exist and one of them may have a chance to be the first to reach "break-even," a key milestone in which a process produces more energy than needed to trigger the fusion reaction.
In American history, Brown evokes a triumphant sense of justice served - a civil rights milestone in which the good guys and the bad guys are clearly discernible.
For preservation advocates, the agreement is a milestone in the way in which cities handle redevelopment, using preservation instead of a bulldozer.
In short, it will be a milestone in the process by which Afghans are finally becoming masters of their own house.
That meeting marked a milestone in Tehran and London's relations, which have been strained since the 1979 Islamic revolution, but Cameron's comments upset Tehran hardliners and embarrassed Rouhani at home.
That civic green light marks a milestone in the planning process, which Um said has been two years in the works.
In our scenario, the formation of a hydrophobic core is a milestone in the folding process which must occur along all the routes that lead this protein towards its native state.
The announcement, which marked a milestone in the dam's progress, was seen in Egypt as a humiliating slap and an indication that Ethiopia has no intention to negotiate over the dam's construction.
It is a milestone in the history of the country, which has known no other leader since gaining independence from Britain in 1980, but activists and opposition politicians described the event as an extravagant waste of money when many citizens are going hungry.
The landmark case, coming after years of dogged work by campaigners, is a milestone in the quest for international justice, which has so often failed to bring to book the perpetrators of some of the most vicious wars in modern Africa.
