As time goes on and the part gets flatter and flatter,
I try to reduce the amount of marking medium and also
to thin it out a bit more with some wd-40 so the
consistency is more even and thin. Then I need to
move the part around more to get a decent mark, but
the mark is still much lighter and shows finer
detail than in the example above.

I've found that most of my mistakes in the beginning
were using the wrong degree of marking, ie. I would
be trying to spot very finely and accurately, when I
had a thousanth or two to move. Or, using too much
marking compound when the part is very flat. Then
it just shows up all blue, there's no contrast.

I have found exactly the same thing. I have just discovered using
oil in the mix, and it makes a big difference. As a beginner, I
never gave much thought to the paint. Paint is paint,right?
But I don't think the batch of paint I have can be spread very very
thin - it is just too thick and clumpy at that level. But with a
drop or two of oil, I can now smooth out the paint very nicely.