import tkinter as tk
from tkinter import ttk

root = tk.Tk()

st = ttk.Style()
st.layout("Vertical.TScrollbar",
          [('Vertical.Scrollbar.trough',
            {'sticky': 'ns', 'children':
             [('Vertical.Scrollbar.thumb',
               {'expand': '1', 'sticky': 'nswe'}
               )]
             }
            )]
          )
bw = st.lookup("Vertical.TScrollbar.trough", "borderwidth") \
     + st.lookup("Vertical.TScrollbar.thumb", "borderwidth")

c = tk.Canvas(root, width=400, height=300, scrollregion=(0, 0, 400, 500))
c.pack(side="left")
c.create_rectangle(0, 200, 400, 300, fill="red")
s = ttk.Scrollbar(root, command=c.yview)
s.pack(side="right", fill="y")
c.config(yscrollcommand=s.set)

root.update()
h = s.winfo_height()
d = h - h / s.fraction(0, h - bw)
print(round(d))
print(d)

root.mainloop()