About 15 days ago I succeeded to follow this almost correct FT8 sequence, but it had a few issues.

Issue
I decoded HB9DGZ answer -05 at 20:05:37 but its transmission was since 20:05:15 till 20:05:27 (my decoding took 10 seconds). 

Therefore (waiting for the even slot) I could plan to send R+06 only 1 minute later, since 20:06:30 till 20:06:42. 

In the meantime HB9DGZ, not seeing my reply, correctly retried answer -05 since 20:06:15 till 20:06:27 which I decoded at 20:06:36. 

At last, at 20:06:30 till 20:06:42 I transmitted R+06

Promptly, HB9DGZ decoded R+06 and transmitted RR73 since 20:06:45 till 20:06:57, which I decoded  7 seconds later at 20:07:04

My upgraded sequence

I answered SV9TLU transmitting JN53                        since 9:33:00 till 9:33:12

At 9:33:15 SV9TLU transmitted -06 to me                    since 9:33:15 till 9:33:27

I decoded -06 before 9:33:30 so I could answer R+06 since 9:33:30 till 9:33:42

Final consideration

Since last SV9TLU CQ (at 9:32:57) till its RR73 (at  9:33:57) elapsed only 1 minute

To see my software improvements, see my my Github