You can type
5% in the text box to the right of the slider,
or run code like this:
Code:
tell application id "OGfl"
set lstSeln to selection of front window
if length of lstSeln < 1 then return
set image scale of item 1 of lstSeln to 0.05
end tell
but I don't think you'll have much luck with changing the slider scale.
If you'd like to suggest a change for a later version, the quickest route is
Help > Send Feedback … from the OG menu.