Acceptable IEEE standard for manufacturing is +/- 7.5% on the impedance, unless specified otherwise.
Meaning the impedance you select, or if none is specified, then the value after testing will be within that range, and you can't say they didn't meet spec.
For a 7.5MVA transformer with a HV side of 13.8kV and below and a LV side of 2.4kV and below, is going to be around 6.75% unless you specify something else; with an X/R ratio of around 14.
The manufacturing tolerance is used in studies where the negative value would be for SC, and the positive value would be for LF, MS, stability, harmonics, and power flow.