We, too, started at an easy place for each child and just did the top passage from each column until they needed 2 days (or 3) to complete a passage for the 3rd or 4th time (we didn't slow down after the first mistake, as we all make mistakes sometimes
). This seemed to be a sweet spot, as they still got most correct on the first try but still got one incorrect after studying once in a while.
You definitely are better to stay on the easy side at first, as going too difficult too soon will lead to problems/frustration. So her choice of #72 may be a good spot. She will still be learning ("how to" for dictation, visual memory, word order, punctuation, etc.), even if she isn't hitting a new spelling word very often just yet.
Also note that for some, the difficulty isn't in the spelling, but in the length of the passages. My "natural speller" started above his siblings (who struggle more with spelling), but found he slowed down when the sentences became longer. Although Carrie has in the guides to read them a phrase at a time, CM advocated a sentence at a time, so that is what we do, and I find that keeps Dictation as not merely a spelling/grammar exercise, but also more of a memory exercise than it would otherwise be, as well.
I love how the same method, the same lesson time (since I have 4 doing dictation at the same time of day), the same material (though at different spots, of course) can be effective for VERY different learning types, skill levels and learning challenges!
Blessings,