I'm not sure if this helps any, but I found it in the Tk docs

When moving up or down a certain number of lines, this is interpreted as logical lines, where each line is terminated only by the "\n". With long lines and wrapping enabled, this may be represent multiple lines on the display. If you'd like to move up or down a single line on the display, you can specify this as e.g. "1.0 + 2 display lines".

I can't get it to work, but I have an older version of Tk. Hopefully it will work for you.