Slivovitz, for instance, is typically 70% alcohol. Proof spirit (100 proof) in the UK is 57.06% alcohol by volume, in the US and Australia it is 50%, in France Gay-Lussac defined proof degree and percent alcohol by volume to be the same.
See https://www.britannica.com/story/why-is-alcohol-measured-by-...
It's always percent ABV (alcohol by volume). As you can see in this table, typical values for rakia (rachiu in Romanian) are 40–86% ( https://en.wikipedia.org/wiki/Alcohol_by_volume ).
Degrees proof is a term originating in English tax law where the drink catching fire was deemed 100 proof. But it's only used in the US now.
A fairly strong one is 42%, same with most other alcohols, even grappa. I’ve had 60% and it was distinctly unpleasant, felt like something you’d drink on a dare instead of for pleasure.
I'd be surprised if the stuff some of my relatives used to distill in Croatia would have less than that lol.
In my experience virtually everyone from the Balkans knows someone who distills themselves or knows someone who does. In my alcohol closet there's like six distinct Slivos from all over South/Eastern Europe. Homemade, of course, and you absolutely do not want to drink more than 2cl... not because of possible methanol but because this stuff will absolutely get you wasted in no time at all.
Where I usually go in the Balkans -- Vojvodina area, if that even counts as Balkan -- the homemade stuff isn't that much stronger than the store-bought. Probably a little, but you end up drinking half a liter of the stuff in a good night, so it can't be much over 40.