It's 128 bytes of source text (the underlying markup language), by the way. Not just the words being rendered on the screen. 1 character = 1 byte (including spaces) is a fairly safe rule of thumb (yes, I'm aware it's not exactly as simple as that for all those perfectionists out there).
I'm afraid there's not going to be any easy or (very accurate for that matter) conversion from file size->source characters->bytes->locations. There's different levels of compression and many mobi's html source isn't as—shall we say—tidy... or KISS as others.
The only way to get the data you really need is to unpack the mobi and gain access to it's raw markup language. Anything else will be basically a wild-ass guess.
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