  We demonstrate that fitted values of stellar radius obtained by fitting
theoretical light curves to observations of millisecond period X-ray pulsars
can significantly depend on the method used to calculate the light curves. The
worst-case errors in the fitted radius are evaluated by restricting ourselves
to the case of light emitted and received in the equatorial plane of a
rapidly-rotating neutron star. First, using an approximate flux which is
adapted to the one-dimensional nature of such an emission region, we show how
pulse shapes can be constructed using an exact spacetime metric and fully
accounting for time-delay effects. We compare this to a method which
approximates the exterior spacetime of the star by the Schwarzschild metric,
inserts special relativistic effects by hand, and neglects time-delay effects.
By comparing these methods, we show that there are significant differences in
these methods for some applications, for example pulse timing and constraining
the stellar radius. In the case of constraining the stellar radius, we show
that fitting the approximate pulse shapes to the full calculation yields errors
in the fitted radius of as much as about +/- 10%, depending on the rotation
rate and size of the star as well as the details describing the emitting
region. However, not all applications of pulse shape calculations suffer from
significant errors: we also show that the calculation of the soft-hard phase
lag for a 1 keV blackbody does not strongly depend on the method used for
calculating the pulse shapes.
