To be honest, there is nor perfect solution for this scenario right now.
You can divide the whole screen of course into 70% top, 30% bottom.
Code:
PANE
ROW
PANE HEIGHT=70%
ROW
PANE HEIGHT=30%
The percentage sizes take respect to the actual content of the PANEs.
You can also force the top part to have a minimum height, so that it does not get shrinked by the bottom part (in case the bottom part has a big content...)
Code:
PANE
ROW
PANE HEIGHT=70%;300
ROW
PANE HEIGHT=30%
This means that the top part will always have a minimum height of 300 - even if its empty and the bottom part at same time containes a lot of content (i.e. the minimum-height is very high).
The combination "one area by default as big as it required to be - but not more than 30%" (so a percentage maximum height), is not really possible right now.
Regards, Björn