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I can have this site generate a random one of 15 words. (Obviously after cloning it from GitHub and moving it into offline computer.) Then presumably I could simply go into "Show entropy details" and add my dice rolls to the entropy until I get up to e.g. 24 words. Anyone here welcome to critique this idea.
That could work too but if you want the final result to be half machine generated and half manual generated entropy then you have to choose 12 words (128 bit) as the start then you'll have to use the same tool to generate the checksum for the final 256-bit entropy.