p. 124, bottom:
It seems that the rounding is not necessary, because scrollbarDrag transmits an integer as long as you don't define a numberformat (or value format depending of how you did set the preferences) in the slider's inspector. To check it, just create a slider with 0 as startValue and 1 as endValue. Don't fill the numberFormat (or, for an equivalent result, set it to 0). You can made the slider as long as you want. Set the handler:
on scrollbarDrag theValue
put theValue into fld 1
When you drag the slider "thumb", field 1 will contain 0 for any position above (or left) to the middle point of the slider, 1 for any position below or right to this point.
Now, if you go back to the inspector and set the numberformat to, say, "0.000", the field will show any value between 0.000 and 1.000, depending on the slider thumb position.
I checked it with LC 5.5.4 and 6 - but if I remember well, the introduction of the numberFormat for scrollbars is relatively recent, this part of the book was probably written with an earlier version (I didn't keep older versions and cannot check it).