How about p. 49 in your own manuscript
Fascinating!
Never heard about any magic number for checking out whether a book starts to "sag in the middle."
Decided to try it with the book closest at hand: my own. Went to the file on my computer. (No telling, of course, how this will correspond to the page numbers of the finished book, when it comes out in December.)
I went to p. 49. And sure enough, I ended up at what I consider a "good spot." A funny episode, way more "showing" than "telling," in a Cajun music memoir that certainly has some stretches that are more earnest in tone. It would be a good spot for a reader to land. Even I had fun re-reading it--and believe me, after endless hours of proofreading, I don't find that true for lots of the book
Just to test it out, I looked at p. 48, p 50. Even p. 41. Definitely not as engaging!
(On p. 49 my husband and I try to test out our Cajun dance lessons at a place where the stakes are high: at a dance hall in Louisiana, our first time there.)
Anyone else want to check out their manuscripts?