I think the 20-25% carry over is probably the best bet. It lets people have a continuation of that character with a new character without feeling like they’ve just wasted a boat load of time and now have nothing to show for it.
Anything more than that really would be a bit off. 20-25% are good substantial numbers because even at 1,000 xp earned that’s 200 – 250xp you gen a newbie character with over other freshly generated newbie characters.
That’s pretty substantial if you think about it.