substituting sweetened chocolate for unsweetened

I have two questions:

First:
It's never been possible to find unsweetened baking chocolate here in Italy. Only recently Lindt came out with a 99% chocolate that works ok as a substitute for unsweetened.  But it costs a fortune.  I generally buy a very cheap brand of chocolate (Novi) which is also my favorite for eating - nice bite, nice clear chocolate flavor.  It says 72% cocoa on the package.  In the English translation of the ingredients it says:
Extra bitter chocolate - Ingredients: cocoa mass, sugar, cocoa butter, fat-reduced cocoa, flavours, cocoa: 72% minimum. 

There are also plenty of other types of "fondente" chocolate here, (fondente means melting - it's the usual baking chocolate, but obviously for italian recipes calling for a proportionately lower amount of sugar.)
One i have handy says: sugar, cocoa mass, cocoa butter, emulsifier soy lecithin, vanilla, cocoa: 50% minimum

Does anyone have a formula for how much more chocolate I would need and how much less sugar I would need to substitute either of these chocolates (72% or 50%) for unsweetened baking chocolate?  I'm absolutely no good at math and can't figure it out for myself.  You can give me weight or volume, I can convert those. 
(And yes, i have looked up on internet, with predictable results.  There were hundreds of sites, many contradictory answers, many very confusing answers, and none out of the first ten or so i found that accounted for the percentages of cocoa to sugar in the chocolate itself).  I'm hoping someone here actually knows the answer and isn't just guessing like some of the sites seem to be doing.)

Second:
I've always been substituting cocoa and butter for unsweetened chocolate; the formula i have is 1 square (ounce, right?) bitter chocolate = 3 tbsp cocoa + 1 tbsp butter, the butter being added with the rest of the butter, the cocoa either added with the flour or with the water.  (I got it in a magazine ad for hershey's cocoa - figured they would want your cakes to come out well so you would buy their cocoa, so they'd make sure to make the proportion right.  It seems to work ok.)
But i wonder if that wouldn't give me a slightly different texture (since cocoa butter melts at a lower temperature than butter, and it might make a difference in the texture of the cake, am i right?)   I am so used to these cakes made with cocoa i don't know if i'm missing something.

 

Thank you so much Tralfaz. 

Reading your reply, suddenly it hit me how stupid the question was.  I've actually been wondering about this for some 30 years.  I somehow thought that the answer would be some complicated formula, not thinking that 50% (or 72% or whatever %) of the ounce would be sugar and the other remaining % would be unsweetened chocolate. 

I think I have my elementary school math teachers and their flash cards to blame for my instant gut reaction when i see numbers, and my mind goes fuzzy in any reasoning process where numbers appear.  I should have thought "half" instead of 50% and i would have seen the light. 

Thanks - the scales have fallen off my eyes!
(pun intended)
siduri