While setting up a list using the seb_blog layout, I've noticed that the width of each column is automatically calculated based on the number of columns in a block.
So, for instance, if TOPBODY-A, TOPBODY-B and TOPBODY-C have any contents, than each column is automatically assigned 33.3% width via CSS (classes cck-w33f,.cck-w34f). If only two columns are present, each gets 50% wide through a shared CSS class (.cck-w50).
After some sleuthing, I found that these automatically generated CSS class assignments could be configured in the "seb_blog - [content_type_name] (list)" template, which is present in the Joomla Templates manager. After changing the values in the Column Width field from "50,50" to "20,80", however, the same .cck-w50 CSS class is till applied, Overriding the value of the width property in .cck-w50 is not a choice because both columns (TOPBODY-A and TOPBODY-B) will use it.
I've set up the number of columns and their width values as per your instruction but apparently the first value (20%) is being applied to both TOPBODY-A and TOPBODY-B, as you may observe in the URL below.
However, mu intention is not to display multiple items in the same row, but one item per row across multiple columns. The problem is that I haven't found a means to assign width values to each column.
If you check http://dev.exponentus.com/ccbulpt/index.php/pt/membros/diretorio again, you'll see 2 items taking the entire row width, which is what I want. But I also need to be able to assign TOP-A and TOP-B custom width values, instead of the values automatically calculated by Seblod (50& / 50% for 2 cols; 33,33% for 3 cols and so forth).
I'm wondering if i should just throw all h fields in the MAINBODY position. and then create the layout I'm looking for through CSS overrides.
I think you are mixing template positions which reffer to the positions inside each item and blog top/middle/bottom settings, which reffer to the layout for the multiple items. Template settings are for multiple items layout.
To style positions inside each item you can use markup setting and add necessary style there, in fact most users nowdays just use minima template for item with all fields in one position and then add bootstrap styles on each field and some sorrounding div elements (or you can use tempalte override for the same thing)