Soft-X-radiation in the "water-window" region (23.3-43.6 Å) mainly from carbon laser plasmas generated by subpicosecond (700 fs) 0.248-μm laser pulses is studied as a function of angle of incidence and intensity (up to 10¹⁸ W/cm²) for p-polarized laser light. Furthermore, comparison is made between plasmas generated from massive and foil targets. Numerical calculations are performed using a hydrocode coupled to X-ray line and continuum emission calculations including radiation transport. The optimized conditions to achieve maximum water-window X-ray emissivity and, in particular, carbon Lyman-α line emission are investigated. In addition, analytical scalings are presented. These theoretical results are essentially confirmed by previous experiments. It is found that at optimized conditions, picosecond or subpicosecond laser plasma X-ray sources with a power of the order of 1-10 GW in a spectral window of 1 Å could be developed.