I definitely feel like rounding too early can be avoided entirely if you understand the 'store' button on some calculators.

The default calculator for the HSC is the Casio fx-82 AU Plus, and if you have the time, definitely consider buying it for the HSC.

The store button, which can be accessed via SHIFT + RCL allows you to store exact values for irrational values, which in turn, avoids rounding incorrectly altogether.