Fun with relative directories...

pathDiff.fs

let GetHintPath hintPathFromRoot root projFile =
    let rootDir = DirectoryInfo(root)
    let projDir = DirectoryInfo(Path.GetDirectoryName(projFile))
    let rec dirDiff (rootDir : DirectoryInfo) (currentDir : DirectoryInfo) levels =
        if rootDir.FullName = currentDir.FullName then
            levels
        else
            dirDiff rootDir (currentDir.Parent) (levels + 1)
    hintPathFromRoot::[for _ in 1..(dirDiff rootDir projDir 0) -> ".."]
    |> List.rev
    |> List.fold (@@) "."

This code is completely untested, so don't rely on it to actually work.

@@ is a custom operator for Path.Combine