The Enterprise's starting position is the only point of uncertainty. Airdate-wise and Stardate-wise, the episode Amok Time (3372.7) happens after Operation: Annihilate! (3289.8). O:A was at Deneva approx. 83 stardates (about one month) prior, providing no idea as to where the Enterprise is in the episode based on how fast the E travels. 
1. WF 4 is ~1.25 x WF 3.5 (13.8 days / 11 days)
2. WF 6 is ~2.75 x WF 3.5 (Enterprise was at WF 4, but only for a short time, so assume the WF 3.5 data; 11 days / 4 days)
3. WF 8 is less than ~2.75 x WF 4. (Enterprise can't make Vulcan then Altair at WF 8; 11 days / 4 days)
Even if the speed conversion for warp is a linear association, this makes no sense. Something else is at play. One possible variable could be in-system speed limits (both physical and administrative) where much of the lapse time is spent at slower speeds traveling into and out of the two solar systems. YMMV
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Also, I guess dropping Spock off along the course in a shuttlecraft is not an option.

Based on your data:Here I’ve guesstimated the ETAs:
WF 3.5 - to Altair - 11 days
WF 3.5 - to Vulcan then Altair - 13.8 days
WF 4 - to Vulcan then Altair - 11 days
WF 6 - to Altair - 4 days
WF 8 - to Vulcan - 3 days
Even if I’m off a bit somewhere, it doesn’t matter, there is no WF scale that we’ve looked at in which there is such a negligible difference in transit time between WF 4 and WF 8.
1. WF 4 is ~1.25 x WF 3.5 (13.8 days / 11 days)
2. WF 6 is ~2.75 x WF 3.5 (Enterprise was at WF 4, but only for a short time, so assume the WF 3.5 data; 11 days / 4 days)
3. WF 8 is less than ~2.75 x WF 4. (Enterprise can't make Vulcan then Altair at WF 8; 11 days / 4 days)
Even if the speed conversion for warp is a linear association, this makes no sense. Something else is at play. One possible variable could be in-system speed limits (both physical and administrative) where much of the lapse time is spent at slower speeds traveling into and out of the two solar systems. YMMV

Also, I guess dropping Spock off along the course in a shuttlecraft is not an option.