Does anyone happen to know what's the mininum character level for starting Act 3 in order to get the option to scale the encounters up for high level parties?

Cool, thanks!

Do you also know whether the scaling is dependent on my current level when I select "High Level" or whether it just gives the opponents a general "High Level" scaling? Like, does it matter whether I choose that option e.g. at lvl 10, lvl 12 or lvl 14?

The scaling depends on *when* it is triggered, not your level. However, you can set the scaler yourself via the console. See here, and two posts down.

What do you mean by "when" exactly? The reason why I'm asking is, I had only just started White March Pt. 2 (choosing High Level, since my party is lvl 14 by now), but then I felt like also moving on to Act 3 already (High Level as well), so that it would be less linear and I could alternate between the two at will. And I was wondering if it would make a difference whether I do that now or completely play through White March Pt. 2 first before moving on to Act 3.

In the meantime, I already went ahead with it, but I'd still like to know whether finishing White March 2 first would have made Act 3 more challenging or not in my case, just for curiosity's sake.

The quoted trigger events.

Oh, you mean the High Level dialogue for White March Pt. 1 will scale the enemies to a different level than the High Level dialogue of Act 3 (or that beyond it), but it will always be a fixed, pre-determinded level? Like High Level for Act 3 will always mean, e.g. lvl 12-14, regardless of whether you start it with a lvl 6 or a lvl 16 character, but it will be different from High Level for White March Pt. 1 which is e.g. always lvl 8-10 or something like that? Is that what you meant to say?

If so, then I guess the way I did it was fine and it wouldn't have made a difference if I'd have postponed Act 3 any further. In any case, thanks for your help!

Precisely. Just a few posts down in the link I gave, the dev. gave the precise scaler variables:
PX1_HIGH_LEVEL (White March Pt. 1)
PX2_HIGH_LEVEL (White March Pt. 2)
ACT4_HIGH_LEVEL (Start of Act 3)
ELMSHORE_HIGH_LEVEL (Jumping into the pit on Burial Isle)

I don't know the figures that the variables carry, but those are what the game uses.