You can use key subtracter by alberto, https://bitcointalk.org/index.php?topic=5360656.0

You could use a known public key and use -a - s with -r 0:f  -n 30 to have 15 + and -15 of your known public key, however use -z and -x to have only x coordinates, then you can convert them all to decimal.
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Great... thanks for this
You are a life saver
I am drastically learning so much as the day goes by...