I think that your method is the best for bumpmaps. DebbieM and me experimented big bugs issues during almost 10 days concerning bumpmaps (textures for the DAZ's dragon model , tested by Debbie). My bum files (converted on my G4) were not compatible, after beeing zipped, with the PC format and in the reverse way. It would be important to communicate this rules for the RR's merchants. Anyway, thank you - I will follow your method.

 
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Holyforest,
Hundreds of shaders for Carrara