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A rule of thumb is one atmosphere above the compression ratio, converted to psi. For the '81 R100, the compression ratio was 8.2:1, so 9.2 times 14.7 is something like 135.
The 9.5:1 was prior to the detuning that began in and around '78. LKChris seems to have this down cold...but I'm pretty sure they detuned the bikes in the early '80s.