An easy way to do evenly divide the wall is to use the crossed diagonals
method of dividing a rectangle in perspective. This looks pretty untidy - it's one of those 'quick and dirty' methods that works, even if it isn't pretty! Getting geometrically correct measurements is a laborious process - for this, we'll go with what looks right. Using the crossed diagonals, find the center of the wall, then do the same for the half each side, and so on.
Keep dividing the wall until you have it divided roughly into squares.