... I have settled on one mile per 1000 ft... mid point...
Sounds like a good rule of thumb, Jerry, though I would qualify it a bit.
The stock SC's sink rate, in round numbers, at 60 mph is 600 fpm and its descent gradient in still air is, consequently, 600' per mile. So 8000' AGL buys about 13 minutes and 13 miles under those conditions. No problem going either way from a midpoint in still air. It should keep your our feet dry even with a 20K component. If more than 20K, however, I'd move my minimum crossing height (or Decision Point if crossing in level flight) closer to the upwind shore, adjust my glide speed, if needed, or any combination thereof. E.g., a 25K headwind will cut my ground speed in half. If the engine quits midway, that will put me in the water 2 miles short of the upwind shore - whichever it is. To stay dry, I could move my 8000' (or Decision Point) 3 miles closer to the upwind shore (time and distance to the downwind shore would not be an issue in this case, even from over the upwind shore). Or I could improve my chances for dry feet by increasing my speed to, say, 95 mph and take the higher sink penalty. At about 1100 fpm sink rate (used here only as an example), I could just make landfall. As an added benefit, I would spend less time in strong headwinds (I get lower sooner), time - and distance - I can use to find an adequate field or shoreline.
A consideration often overlooked is the climb gradient to the planned crossing height under the conditions - e.g., climbing on course over water in a tailwind. If the climb gradient is less than the estimated worse case decent profile to at least one adequate field, then a racetrack climb will be needed to stay at or above at least one of those profiles throughout the climb. As an aside, planning the decent to a field near or on an upwind shore is also worth the effort. Starting down early (e.g., to bring CHTs down slowly) in such cases is to court Murphy's law, too. The smart money says descend in a racetrack, if needed, staying within gliding range, preferably upwind, of the field.
Don't get me wrong. I don't suggest these mental gymnastics at a time when stress levels are already high. The point I'm trying to make is that ball park figures ought to include validity limitations when warranted. So my suggestion is to append "... for wind components up to 20K. For on course climb, 600 fpm ROC minimum and no tailwind" to your rule of thumb. (I use ROC here instead of climb gradient because it's readily available off the VSI and always meets or exceeds the required 600' per mile climb gradient.)