Gerdo,
1) Swapping the way the gas springs are mounted would make very little difference to how strong they need to be. Here is the example above with the springs mounted both ways:
How effective the gas spring is in holding the hatch open depends on how far out from the hinge it works. This is measured at right angles to the line of the spring. You can see in the two cases above, the difference is negligible. I've checked all the intermediate positions and there's no difference there either.
That wouldn't necessarily be true for a different hatch design, but I don't think the difference would ever become huge.
What is different is that now, in the right example, there's three-quarters of the hatch hanging out past the gas spring, compared to only half the hatch in the left example. So the bending in the hatch ribs is more than doubled - though whether that's significant depends on how strong they are!
2) Your hatch weighing method sounds fine to me, as long as it was perfectly level when you weighed it, and the prop was right at the end of the hatch.
3) As I understand it, your biggest loading is a hatch weight of 60lb and a distance to the hatch's centre of gravity of 30", so that's a 1800lb-in closing moment.
Divide that by 12" for the lever of each spring, and by 2 for two springs, and you get 75lb force on each spring.
110lb springs would give a 50% margin on that.
Roughly (because I don't know your hatch's geometry) you would have to pull down with a force of 14lb on the end of the hatch to close it. With 120lb springs that force goes up to 18lb and with 150lb springs it goes up to 30lb.
So even with 150lb springs, double the 'hopefully just enough' level, you're not going to be hanging two of you off the end of the hatch to close it!
Andrew
PS I was thinking of changing my avatar to a 'boffin' (you know, white lab coat with plenty of pens in the top pocket, and a forehead at least 12" tall polished to a fine shine), but I can't find one on the web (and I look more like the missing link to the apes, so a self-photo is out).