EGCJFlyer,
I'd echo Whirlybird's and Genghis's comments ref. flight planning. You're obviously doing your calculations carefully, and taking it seriously, but there cannot be too much care taken - as you've said.
I know it seems a logical impossibility to plan for the unexpected, but as an example, you've planned W&B, you've calculated fuel burn, you've looked at alternates. You must also plan for a vertical profile which does not match your estimates; there might be a lot more climbing, etc., for example, than you want - owing to weather or other factors. Allow, also, a margin for the aircraft weight to have increased slightly since the last weighing.
I also fly from Sherburn, and I know the Warriors (but, with five of them, I'm not sure which you are going for, though the probability is one of the ex-CSE aircraft).
So, plan for the not quite expected. Especially take seriously the impact of fatigue (decision-making fatigue, as Whirly has mentioned).
Above all, and I'm sure you're going to do this, talk it all through with one of the instructors first.
(I'd be looking (if I have four up) to book the Archer for a trip like that - but then, I'm fairly heavy!)