Text styles: character size overrides line height

Hello forum,

I am dealing with a pamphlet right now and I have been having a hard time with text styles namely to get the text to fit on a baseline grid.  The grid is 12 points and so I have designed the styles so that all line spacing properties are multiples of 6, so that I can get in-between spacings between headings, and then just throw in a line  with no text, styled with 6 points specified before and after paragraph as well as line height.

Now, this sounds good in theory, but in practice it is a lot harder. I found out after many trial and errors, that the character size also counts and overrides the line height which makes the whole thing of creating text styles a total guesswork.

For example, the main text is Sabon 10 pt with 12/12/12 (berore, after, line height). Now I want to add a paragraph heading with Myriad and I make it 10,5 bold, to make it stand out a bit, with the same line spacing. Now this will wreck the whole thing so that lines in different columns don't line up because the text will deviate from the baseline grid.

When I change text size of the paragraph heading to 9,5 pt it suddenly behaves and fits in. Now, that is good, but I don't think text styles should behave like that, they should respect the line height and if the text is too big, the letters on different lines should just overlap. 

Now when the text size matters and overrides line height (at least in paragraph text) it makes the whole thing unnecessary complicated and difficult. I hope you can look into this Corel.