[ft] Freetype vs. FontLab values for Linegap

[Юлиана Зигангирова]

Hello everyone!

I need to investigate a value for a typographic line gap
for a given font. With "sfnt" fonts I just open OS/2 table and
use "os2->sTypoLineGap", but I have a problem when dealing
with Type 1 fonts. 
As far as I understand, Type 1 fonts theoretically do not
have any OS/2 table; but if I open a type 1 font in FontLab
I still see the "sTypoAscender", "sTypoDescender" and "sTypoLineGap"
values under FontInfo / Metrics and Dimensions / True-Type-specific 
metrics / os/2.
For examples, for URW-Fonts which comes with Ghostscript I see 
values like 
TypoAscender     624
TypoDescender    -205 
LineGap          62   
or similar numbers.
Ascender and Descender corresponds to the values which I get from
.afm file, but where on earth does FontLab take the value for LineGap from  -
like 59, 62, 67 etc.?
When I try to substract typoAscender and typoDescender from a "height" value,
which FreeType gives me (face->height, usually 1200 for these Ghostscript
fonts or such...), I get much bigger values than this. 
When I substract "hhea"  ascender and descender from a height value -
the result is still different.
Does Type 1 have some additional metrics info which I do not get with
Freetype? 
How else could the value for line gap for Type 1 font be calculated?
I hope very much on your help,
Yuliana