Tom, if I understand correctly, you will be using a top/head tube lug with twin sockets to accept twin laterals to make your wife a mixte. The height of this lug above the down tube lug will be controlled by 2 factors. 1st, where the twin socket angle is best situated to aim them directly to the dropouts. And 2nd, where the height of the twin laterals are low enough to comfortably step over but not so low as to lose support. In other words higher is better than lower but those 2 factors push the laterals lower.
Now where your top tube lug is located may be a bit low so too much quill stem or stackers are used. Of course it is possible to extend the head tube above the head lug as long as you reinforce it. This is where you use a section of 1 3/8" OD tube with a wall thickness of .083". This is necessary when using a 1 1/4" head tube with .9 or 1.0mm wall thickness. When the head tube is reamed to fit a normal head set, the wall becomes too thin and might distort under normal use. If you add the sleeve above the top tube lug, it doesn't matter how long it is. The length you choose will be based on where your stem is located. Again you don't want too many stackers if using a threadless steerer or stem extension if using a quill stem.
Usually the sleeve is filed to blend in with the top tube lug. Sometimes the sleeve is shortened so it is located only at the top and there is some space between the bottom of the sleeve and the top of the lug. I'm not a fan of that look but in reality it doesn't look bad.