  We study theoretically current quantization in the charge turnstile based on
the hybrid (SINIS or NISIN) SET transistor. The quantization accuracy is
limited by either Andreev reflection or by Cooper pair - electron cotunneling.
The rates of these processes are calculated in the ``above-the-threshold''
regime when they compete directly with the lowest-order tunneling. We show that
by shaping the ac gate voltage driving the turnstile, it should be possible to
achieve the metrological accuracy of $10^{-8}$, while maintaining the absolute
value of the quantized current on the order of 30 pA, just by one turnstile
with realistic parameters using aluminium as superconductor.
