What's the resolution of timezone8

[ianpengGithub]

Thank you very much for the awesome project but I have a question about database resolution.
I need to get the result in less than 1ms on my platform, so I built a timezone database with resoultion "8", the performance is acceptable now, but it seems that almost all of my country is on the border, as shown below, it is said that the resolution of timezone16 is ~0.5km, what's the resolution of timezone8?

./demo timezone8.bin 24.83111730508183, 121.00844952512162
Target point is border vertex:
  meta: 5928
  polygon: 385
  TimezoneIdPrefix: Etc/
  TimezoneId: GMT-8
  CountryAlpha2:
  CountryName:
Target point is border vertex:
  meta: 4237
  polygon: 614
  TimezoneIdPrefix: Asia/
  TimezoneId: Taipei
  CountryAlpha2: TW
  CountryName: Taiwan
Safezone: 0.000000
The simple string is [Etc/GMT-8]