In an experiment of measuring the refractive index of a glass slab using travelling microscope in physics lab,a student measures real thickness of the glass slab as `5.25mm` and apparent thickness of the glass slab as `5.00mm`.Travelling microscope has `20` divisions on vernier scale is equal to `49` divisions on main scale.The estimated uncertainty in the measurement of vernier scale is equal to `49` divisions on main scale.The estimated uncertainty in the measurement of refractive index of the slab is `(x)/(10)times10^(-3)`,where `x` is _______