How do I ensure enough space between scores?

[Nah]

Lilypond sometimes wants to compress too much. I've been adding
\markup { \vspace #1 }
between scores. It generally works well enough, but seems a crude hack. 
I have to increase the \vspace or add other dummy markup when I have 
text immediately preceding a score (e.g. instructions or a description 
in an lesson book). What I want to do is tell lilypond to give me a 
minimum (let's say 1/2") between lessons. The default spacing within the 
lesson seems fine. I just need to make sure that the lessons are set off 
from each other with sufficient whitespace.

Each lesson is in its own file and organized as:
1) sometimes a title
2) sometimes some text
3) always at least one score
2) possibly other scores (e.g. for a 2-movement sonatina)
4) possibly some more text

In the main file I have:
\include "L01.ily"
\allowPageTurn
\markup { \vspace #1 }

\include "L02.ily"
...


BTW:
What does this mean?
"programming error: found a page-turnable place which was not breakable" 
followed by "continuing, crossing fingers"