Notes

So, at 1920x1080.  Let's assume you want reasonably readable text, but a good-sized dialogue box.  You said 30% of the screen, so that's 324 pixels in height, assuming a fixed box at the bottom of the screen.  Let's use a 36-pixel height font, that's nice and readable.  Also gives us 9 lines of dialog, quite a bit.  This is assuming dense text drawing.  That is, single-spaced, no margins on the sides of the text dialogue box.

So, you write a long bit of dialogue, and you reveal something big in the dialogue.  It fills up all 9 lines completely, end-to-end on the screen.  Great.

Now we have user A, who is using a 16:9 screen.  This'll appear just fine, whee.

Next, user B, who is using a 4:3 screen.  Amazingly, these still exist, and people still fucking use them.  Heavens knows why.

Let's assume that user B is using a 4:3 resolution with the same height as the 1920x1080 screen.  So, 1440x1080 resolution.

Suddenly, you have 480 less pixels to work with horizontally.  You can still display 9 lines of text, because the height is the same.

But the length of each line is now 25% shorter.

Your text now has to be split into two chunks that are displayed sequentially, because the text doesn't fit in those 9 lines at 1440 width.

Your big reveal gets cut off mid-setence, and ends up on the second chunk of dialogue.

See the problem?

You might think this is not an issue, but in writing, this can be a bit jarring.