[XeTeX] line spacing

[Yves Codet]

Hello.

I'm still trying to adjust parameters with Zapfino. I noticed something 
  in a log file about which I'd like to inquire:

% deleted
[2]

LaTeX Font Warning: Font shape `U/Zapfino+scale:0.66+18,0+21,0/bx/n' 
undefined
(Font)              using `U/Zapfino+scale:0.66+18,0+21,0/m/n' instead 
on input
  line 15.


Underfull \vbox (badness 10000) has occurred while \output is active [3]
Underfull \vbox (badness 10000) has occurred while \output is active [4]
Underfull \vbox (badness 10000) has occurred while \output is active [5]
Underfull \vbox (badness 10000) has occurred while \output is active [6]
Underfull \vbox (badness 10000) has occurred while \output is active [7]
Underfull \vbox (badness 10000) has occurred while \output is active [8]
Underfull \vbox (badness 10000) has occurred while \output is active [9]
Underfull \vbox (badness 10000) has occurred while \output is active 
[10]
Underfull \vbox (badness 10000) has occurred while \output is active 
[11]
Underfull \vbox (badness 10000) has occurred while \output is active 
[12]
% deleted

In "scale:0.66+18,0+21,0", "0.66" is the result of my command 
"\setromanfont[Scale=0.66]{Zapfino}". What are the other two values 
(line spacing and...)? Is it possible to modify them with a command 
such as "\setromanfont[Scale={0.66,18.0,21.0}]{Zapfino}" (with 
different values, of course, since I'm trying to remove all those 
"Underfull \vbox..." and to get a nice display of the text)?

Thanks in advance.

Best wishes,

Yves