Sub Articulo 39.1
The baseline that is used is the baseline in effect when a paragraph
ends. I would guess you are ending the scope of your baselineskip change
before the blank line that ends the paragraph, right?
Look at the following example:
1 ...text before the heading.
2 {\Large Heading That Goes on Long Enough
3 to Require Two Lines
}
4 Additional text that follows the heading...
Assume that \Large sets the font size to 12pt and the baselineskip to
17pt, and that the other text uses 10pt/14pt. Then the baseline of
line~2 will be 17pt below the baseline of line~1, the baseline of line
~3 will be 17pt below that, and the baseline of line~4 will be 14pt
below that. This assumes that the height and depth of adjacent lines do
not add up to more than the relevant baselineskip.
If the heading uses up only one line, there will be 17pt
(baseline-to-baseline) above and 14pt (again baseline to baseline
below). This ammounts to 21pt. If you want the total space used to
equal 28pt, then you simply add 7pt of space and divide it as you like
above and below.
I have done this a number of times. You can create a box of a particular
height in the following way
\vbox to 50mm{
\vskip 20mm
\hbox{Chapter 1}
\hbox{Introduction}
\vfil}
In this example, the first \hbox will start 20mm below the the top of
the \vbox, and the \vfil at the end will stretch to whatever is needed
to ensure that the \vbox is 50mm tall. The two \hbox'es are placed with
the baselineskip set previously. (One could omit the \hbox
wrapping, simply forcing a linebreak between the two lines, but I find
it easier not to have to worry about all of TeX's myriad paragraph
building parameters.)