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@gbj_tester
No, I mean exactly what I said.
1 extra tooth = +0.5" circumference
180deg chain wrap, so extra chain needed is half the change in circumference = 0.25"
Extra chain length is taken equally from upper and lower runs, so change in effective chainstay length is half the extra chain length = 0.125"
This approximation is exactly true when the chainring and sprocket are equal sizes, but the error introduced by the chain being angled and the wrap being not exactly 180deg is trivial for normal chainstay lengths and bicycle sprocket/chainring sizes.