Try to be more precise in your measuring. Use an instrument with a smaller resolution, and read it to the smallest reading possible.
Ruler A will give a more precise reading and will reduce the uncertainty in your result.
An uncertainty of 1 mm in 25 mm is 4 %.
An uncertainty of 1 cm in 2.5 cm is 40 %. This is ten times bigger.
The resolution is the smallest division on the instrument.
What is the uncertainty of a resolution of 0.01 ml in 5.52 ml?
(0.01 ÷ 5.52) × 100 % = 0.18 %
What is the uncertainty of a resolution of 0.5 ml in 5.5 ml?
(0.5 ÷ 5.5) × 100 % = 9.1 %