How control the value of a tkinter `Scale` widget depending on the value of another tkinter `Scale` widget in Python?

I have 3 Scale widgets scale1, scale2 and scale3. I would like to use scale1 to control the values of scale2 and scale3 in the following manner:

• I don't want scale2 and scale3 having the same value as scale1.
• If value of scale1 is 1, value of scale2 is 2.5 and value of scale3 is 1.5
• When I move scale1 to the right to increase its value to 1.05, the value of scale2 will be 2.55 and scale3 will be 1.55
• Or I move scale1 to the left to decrease its value to 0.95, then scale2 value will be 2.45 and scale3 value will be 1.45

Here is my code:

from tkinter import *
from tkinter import ttk

class Init(object):
    def __init__(self, master):
        self.master = master
        self.master.title('Scale demo')
        self.master.configure(background='#ededed')
        self.scale_1_var = DoubleVar()
        self.scale_1_var.set(1)
        self.scale_2_var = DoubleVar()
        self.scale_2_var.set(1)
        self.scale_3_var = DoubleVar()
        self.scale_3_var.set(1)

class Scale(Init):
    def __init__(self, master):
        Init.__init__(self, master=master)

        self.frame_master = ttk.Frame(self.master, padding=(10,10,10,10))
        self.frame_master.pack(fill=BOTH)

        self.label_1 = ttk.Label(self.frame_master, text='Scale 1:')
        self.label_1.grid(row=0, column=1, sticky='w')
        self.scale_1 = ttk.Scale(self.frame_master, orient=HORIZONTAL, length=400, from_=0, to=4, variable=self.scale_1_var)
        self.scale_1.grid(row=0, column=2, sticky='we')
        self.entry_1 = ttk.Entry(self.frame_master, textvariable=self.scale_1_var, width=3)
        self.entry_1.grid(row=0, column=3, sticky='e')

        self.label_2 = ttk.Label(self.frame_master, text='Scale 2:')
        self.label_2.grid(row=1, column=1, sticky='w')
        self.scale_2 = ttk.Scale(self.frame_master, orient=HORIZONTAL, length=400, from_=0, to=4, variable=self.scale_2_var)
        self.scale_2.grid(row=1, column=2, sticky='we')
        self.entry_2 = ttk.Entry(self.frame_master, textvariable=self.scale_2_var, width=3)
        self.entry_2.grid(row=1, column=3, sticky='e')

        self.label_3 = ttk.Label(self.frame_master, text='Scale 3:')
        self.label_3.grid(row=2, column=1, sticky='w')
        self.scale_3 = ttk.Scale(self.frame_master, orient=HORIZONTAL, length=400, from_=0, to=4, variable=self.scale_3_var)
        self.scale_3.grid(row=2, column=2, sticky='we')
        self.entry_3 = ttk.Entry(self.frame_master, textvariable=self.scale_3_var, width=3)
        self.entry_3.grid(row=2, column=3, sticky='e')

        self.scale_1.config(command=self.sc1)

    def sc1(self, *argv):
        self.a = self.scale_1.get()
        self.scale_2.set(self.a)
        self.scale_3.set(self.a)

root=Tk()
Scale(root)
root.mainloop()

here is what i've done:

def sc1(self, *argv):
        self.a = float(argv[0]) 
        # rather than re-request the value resulting in more function calls
        # i'm using the value that is passed in during the callback
        if not hasattr(self, "a_prev"): # this is a hacky method to 
            self.a_prev = 1             # remember the previous value

        b_diff = self.scale_2.get() - self.a_prev 
        c_diff = self.scale_3.get() - self.a_prev
        # calculate the differences to the previous value

        self.scale_2.set(self.a+b_diff) # set the scales
        self.scale_3.set(self.a+c_diff)
        self.a_prev = self.a # remember current value as the new previous