Hi Meatbomber,
Yes I was afraid you'd ask me that!
To use your example winds aloft are less than 20 kts, surface winds are 15-20 out of the north and the DZ is oriented east west trees either side (seems like that always eh?)
A .4 .5 offset for what are pretty high surface winds would work nicely, a westerly jump run distributes everybody a long a line upwind of the peas, allowing everybody to make it in but like you say the offset complicates the exit point.
At any given time you'll never get any closer than .5 nm or "abeam" to your waypoint so think of .7 prior as being .2 prior and say .8 past as .3 past you desired exit point etc. The amount of distortion mathematically is negligible because of the relatively small distances involved. These distortions however magnify if you get into offsets bigger than .7
So you want to distribute the jumpers equidistant EITHER SIDE of the abeam or .5 offset you plan to fly, so as to keep everybody as close to the DZ as possible. Exception winds aloft >20 then you go deeper.
The success of your offset jump run is also dictated by the Update rate of your GPS.
Again the critical element is the number of groups as you know 12 groups of 24 freeflyers is a challenge to get everybody in to the DZ and I have in the past made two jump runs to facilitate these situations. (boss didn't like that)
To re-cap winds aloft <20 will have little effect on freefall drift (except large rw formations), The amount of offset will be dictated by surface winds, try .5 for 15-20 kts and adjust up or down as necessary, be wary of offsets greater than .8
The main thing is to fly the PLANNED jump run and get feedback so you can dial it in as necessary. It's a bit of a black art If everybody makes it in on the first load of the day you're the man!!
With reference to the WDIs as you say they are useful when doing static line drops from say a Cessna but for operational and economic reasons not practical in large aircraft skydive ops.
Regards
Leviathan