And if you use a sliding bar for that value? with a little space where show the current value? If you 've been told to not validate just constraint!

just to give another perspective to your conundrum! (nice word anyway..)
I think this is named 'scale' in Tk world:Tk::Scale

$parent->new_ttk__scale(-orient => 'horizontal', -length => 200, -from
+ => -10.0 -to => 10.0)
[download]

L*
PS: tkdocs