The out of the box layouts have "fixed" CSS.
One of them is indeed 50% 50%, and there is another one that got added in 3.1 (IIRC) which allows to adapt the size of the first column to its content (and the other column takes what's left). I forgot the exact name, but you can find it in the page composer.
With the out of the box layouts there is no 30%/70% ones, you would have to provide your own, but I don't think that this is documented somewhere except in a post in that forum.
Cool, the one which allows to adapt the size of the first column to its content (and the other column takes what's left) is exactly what we would need , FYI its the "Autotfit Column Layout". The trick is that it shows the new layout only when the page is saved, confusing the user who thinks that the configured layout was not working properly. When you save the page , everything is ok.