I noticed a minor mistake: the default is (mostly) equivalent to to '(0.5, 0.0, 1.0), i.e. the first two numbers are swapped.
I say "mostly", because there is one rather annoying difference between 0.0 and top, and 1.0 and bottom respectively: recenter-positions isn't used only for Ctrl+L (recenter-top-bottom), but also Alt+R (move-to-window-line-top-bottom). When using 0.0 and 1.0, the latter scrolls the window for some reason instead of just moving the point.
I'm now using '(0.35 top bottom) and am much happier with it than the default, thank you for the pointer! :)
Another tip: you can still center the current line by prefixing, i.e. entering Ctrl+U Ctrl+L.