Karin: Nice work on the Ingres copy. You are doing the same type of underpainting as we are, you just aren't using our fancy name!
BTW: Titanium white is very fat. It takes a ton of oil to make titanium into paint. Your process might be better served if you considered swapping titanium out for a lead white - either Flake or Crementz. A lot of people have reservations about using lead paints, but if you use a modicum of care, they really are just fine. They dry faster and are better for the painting in the long term because you are putting a lean layer under rather than a fat layer.