relative.swift

extension View {
    func vertical(_ key: VerticalAlignment, relative: CGFloat) -> some View {
        self.alignmentGuide(key, computeValue: { $0.height * relative })
    }
    
    func overlay<V: View>(alignment: Alignment, relative: UnitPoint = .center, _ other: V) -> some View {
        self
            .alignmentGuide(alignment.horizontal, computeValue: { $0.width * relative.x })
            .alignmentGuide(alignment.vertical, computeValue: { $0.height * relative.y })
            .overlay(other, alignment: alignment)
    }
}

struct ContentView: View {
    var body: some View {
        Circle()
            .frame(width: 200, height: 200)
            .overlay(alignment: .center, relative: UnitPoint(x: 0.3, y: 0.3), Circle().fill(Color.red).frame(width: 30, height: 30))
            .overlay(alignment: .center, relative: UnitPoint(x: 0.5, y: 0.75), Text("Hello").foregroundColor(.white))
    }
}

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}