What do you mean with "monitoring the same Server"? As I already mentioned, the host-header normally is only required if you are working with virtual hosts on your server or if the "listener" on your server requires explicit a host-header. Again, if you are unsure how your send-string should look like, please try it manually via CLI and verify how your server/application behaves.
Regarding the regex for the receive string, please check this website with all official HTTP Status codes. As you can see the "OK" belongs explicit to the 200 code, means there isn't an official "300 OK". Further details on the correct syntax can be found in this solution.
Ciao Stefan :)