Even if you go with the ems equivalent of 42px, that's a heck of a lot of wasted screen space. I've seen a lot of eBooks using 10% or 14% for chapter titles for a heck of a lot of wasted space. Not only is the screen space wasted, it also looks awful. I [refer a small about of space at the top followed bu the chapter header and then a small amount of space and the chapter text.
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