73.276% is the scientifically proven place to start. This is assuming it's been exactly 6 months, down to the minute, since you left off - I assume you chose your words with due scientific precision. If it was 6 months and 3 days, or even worse, 6 months, 1 week, and 4 days, the algorithm is completely different, naturally.
I kid, I kid. The best thing to do is exactly the protocol described in SS for your first ever workout. It should be heavier than you actual first ever session, since some of that strength will remain even after 6 months off, but you do the same thing: work up to a moderately heavy set of 5 where the bar speed just barely starts to slow down involuntarily on the 5th rep, and you stop there - that's your working weight for that lift, and LP up from there. Assuming you haven't lost a lot of weight, or if you have but are willing to put it back on fairly quickly, you can usually handle bigger jumps back up for a while - no hard and fast rule, but at least a few workouts, and often till you're pretty close to where you were before, before switching back to small jumps.
A hypothetical example, that may not exactly be applicable but can illustrate the concept:
Previous squat: 295x5. New moderately hard squat found on first day back: 185x5. Workouts:
1. Work up to 185x5 and then stop
2. 185x5, 205x5
3. 205x5, 220x5x2
4. 235x5x3
5. 245x5x3
6. 255x5x3
7. 265x5x3
8. Reintroduce light squat midweek at 210x5x2
9. 275x5x3
10. 285x5x3
11. 215x5x2
12. 290x5x3
- so you hit your old number in about a month, whereas it probably took you around 3 months to get there the first time making mostly 5 lb jumps after a few initial bigger ones. If it took you only 1-2 months the first time, you can probably make more big jumps here too, and get back sooner than a month.