Use dynamic text size .largeTitle but always 18% larger than normal in SwiftUI

Question

I want to use dynamic text sizes in the system font. But our design wants the default large title to be a bit bigger than the 34pt Apple default.

How do I make my SwiftUI style .font(.largeTitle) relatively larger, i.e. 40pt at system default text size, but still scale proportionately when the user changes the system text size? Always 18% bigger than a normal .largeTitle.

I don't want to specify the San Francisco font name using .custom(<name>, size: <Float>, relativeTo: <Font.TextStyle>) as this generates a warning and misses out on getting the best optical variant.

I can use UIFont functions to adjust font metrics but I cannot convert the too-small SwiftUI Font to UIFont.