If it's a perfectly rectangle frame with W = 50 cm & L= 90 cm
So the right diagonal should be √(50² +90²) = √10600 =102.96 cm
as per the calculation the diagonal should have a length of 102.96 cm, but the boss found 100 cm, that means their should be a frame which diagonal equal to 100 cm, short by 2.96 cm, that means short by 2.96/2 = 1.48 cm from the bottom & 1.48 cm from the top (same as well as for the 2nd diagonal)
In this case the boss should be happy