I use custom printer profiles, which give perfectly adequate soft-proofing for whatever paper I'm printing with. Your method offers no advantage over the plugin's full-screen preview, since you can't actually soft proof to the final output device until after you've converted the file to the QTR profile and back to a standard RGB or grayscale working space, which BTW is a procedure likely to produce more posterization/quantization errors than theimagingfactory's plugin or the Channel Mixer, which offers as much control over B&W conversion as your method in a single simple step far less likely to introduce image artifacts than your unnecessarily convoluted method.