Mulling some stuff.

1) Jump is precise: the canonical variation of a day or so MUST be something that the navigator knows ahead of time, as part of calculating the jump, because that space of a day will mean that the target system's star, planets, everything will have moved. If the canonical ability to jump within 100 diameters of a specific planetary body is correct (as well as other canonical references, like the HG allusion to coming out of Jump with black globes on to sneak beyond and surprise an enemy fleet) then the duration of a jump MUST be a known factor. It needn't be known until calculation of the jump is complete, but it will be known afterwards.

2) It's been established that jump masking is a thing that happens. That means that solar diameters, GG diameters all make a difference. Needn't figure that stuff out for every system, but for some? It would be a good idea.

EDIT: Setting up whole systems via Book 6 has been annoying, persnickety, and revelatory. It's cool: adds some real "geography" to space that'll be fun to play out. If you go to the trouble of figuring out orbital periods and calculating the changing distances between planets and satellites, it gets even better: In the space of weeks, the relationships between every body in a system might change, with effects on politics, trade, everything. It's cool.

3) I really ought to figure out a way to rough out typical relative velocities for starship encounters if I'm going to continue putting up with vector combat. If one goes with High Guard, that's not so necessary:

EDIT: Keeping this simple. A simple encounter:
a) set location of ship A, choose direction of ship's destination and determine distance from it (for ex, 640000km from mainworld)
b) choose range and bearing of ship B (appearing at detection range, direction 1-6 from ship A)
c) choose relative vector of ship B (1-6+ vector, direction 1-6)

Begin play.

4) If you're putting out a distress call, how soon can one expect help? One model I toyed with is this: each turn of combat, roll 2d against planetary population, with a modifier for starport type: if the result is population or lower, there's a response and help is on the way - in 1-6 turns. (Is that too soon to hope for? Maybe it matters where in the system the encounter takes place?) On the turn help arrives, it shows up in the reserve: on subsequent turns, the assisting ship can take the line.
EDIT: So far this seems to work for me.