I've been thinking about this for the weekend. When using a microwave oven I will often enter 1:11 or 1:33 instead of 1:10 or 1:30 just because it is easier.

I think that since the number of words within a sentence is quite small on a Kobo that the binary search for the word within the line is not necessary.

Originally I was worried that switching methods from binary for finding the line to linear for searching a word might be too disruptive but since I also suggested going from a full underlined sentence to underlined word I think there will be no problem.

I still think that a binary search to find the line is useful. I checked a document at Medium and Smallest font sizes and found there were 20 and 30 lines respectively.

With a binary search the maximum number of clicks to find a line would be 5 for each example (actually 4.high-decimal for 30 lines or 4.low decimal for 20 lines but you can't have decimal click). If you just linearly click up or down to the line then with the 20 line example 50% of the time you may need to exceed 5 clicks and for the 30 line example it becomes 75%.