aef110, you can find values for the distances of each frame edge in the Properties panel at the bottom of the window. (If in doubt, check the Properties panel - it’s often the answer!) For most topics about moving things, I include a tip at the bottom about which properties control their offset, but this is indeed missing from frame-related topics. I’ve made a note, thanks.
For centering a subtitle, I’d probably recommend making the frame the full width of the page and using the text editor options (or indeed the relevant paragraph style for your subtitle) to center the text. (I see you’ve already done this - I’d still recommend this way as it gives you more flexibility if you e.g. change the subtitle and it gets longer, meaning if you’d made the text frame only as big as required you’d probably have to change it. You can also enter text frames by selecting any part of them, including a handle, and pressing Return.)
These sorts of more specific examples are on my list to work into the docs at some point - because you can do literally almost anything with frames/text/alignments, the general documentation does have to be quite general, but rest assured I do have plans to improve support for common operations.