Following a suggestion by Joe Kelchner, ("I suggest breaking down each thing to it's absolute minimum and then model it in a two dimensional sketch and get that to work like it should and then use that as a layout sketch"), I am creating a complete 2D sketch of my new project before doing any 3D work.

On the other hand, Jerry Steiger and told me my sketches should be fully defined (https://forum.solidworks.com/message/164513#164513). Attached is a sketch that is underdefined--select for instance the simple inner ellipse in White-SCF-Dura-Ambient (this is the white matter in the spinal cord). Why is it underdefined? How do you diagnose underdefinition? How do you fix it?

Taking a blind stab at defining the sketch further, when I try to fix its center to the origin (which I thought was already done when I created it), it becomes over-defined.

My texts do not really address fully defining a sketch and I had thought it was a good thing to be underdefined because clearly I get error messages when a sketch is overdefined and sometimes have to wade thru many options to fix the overdefinition. Here is what my best text, Planchard's A Command Guide for SolidWorks 2009, says, which seems ambiguous. I know I was naive in thinking an underdefined sketch is good--I just want a set of best practices for fully defining a sketch. Thank you.

"...it is not necessary to fully dimension or define sketches before you use them to create features. You should fully define sketches before you consider the part finished for manufacturing." (p. 4-17).

btw all suggestions I have received on this forum have been very helpful--I really appreciate them.