Do the calculators replace the datasheet?
No. They are quick tools for getting an initial value or checking a
calculation, but the datasheet remains the reference for electrical limits,
temperature, tolerance, dissipation and operating conditions.
Why does the standard value not always match the ideal one?
Components are sold in normalized series such as E12, E24 or E96.
When the theoretical value does not exist, choose the nearest standard value
and check whether the error is acceptable for the circuit.
What power margin should a resistor have?
As a rule of thumb, use a resistor with a power rating
at least twice the calculated dissipated power, especially if the
component works in a closed enclosure or near heat sources.
Do the formulas also apply to real components?
The basic formulas are correct, but real components have tolerances and
non-ideal behaviors. Capacitors and inductors, for example, change
behavior with frequency, temperature, current and applied voltage.