Tkinter percentage sizes?

I am trying to build a small software with the Tkinter module and python but I can't figure out how to set a widget size with percentages instead of pixels. I could of course do it by grabbing the size of the entire window at every moment and divide by 2 to get a size of 50%, but is there a better way to do it with the Tkinter module?

EDIT 1: To answer your questions I put my code here:

class Navbar:
    def __init__(self, master):
        self.maxsizeFrame = Frame(master, height = 50) #maxsize handler, prevent self.frame to grow too big
        self.maxsizeFrame.pack(fill = X, expand = NO)
        self.frame = Frame(self.maxsizeFrame)
        self.frame.pack(side = TOP, fill = BOTH, expand = YES)
        self.masteries = Button(self.frame, text = "Masteries")
        self.masteries.pack(side = LEFT, fill = BOTH, expand = YES)
        self.runes = Button(self.frame, text = "Runes")
        self.runes.pack(side = RIGHT, fill = BOTH, expand = YES)

So I want to create a navigation bar with two button handled in "self.frame". But I want this design to be responsive, that's why I set expand to "YES". However I wanted to set a maximum size for "self.frame" but the only way I found was to pack this frame in an other one ("self.maxsizeFrame") and set expand to "NO" on this one. And finally, I would like to set the maximum expand size to half the main window, so it could be great if the height of "self.maxsizeframe" could be in percentage. Thanks for reading.

EDIT 2: Actually it seems more efficient to build the software with the grid layout and the weight's option would be accurate.