My current plan is to leave the demo with at least 70% of the content (same strategy as my previous project), so players can fully understand what they are buying.
But I can't promise you that this plan won't change in the future, because psychologically people may feel like they paid full price for 30% of the content just to see the ending. If so, I might flip that portion to 30% free demo and 70% paid content.
For your information, the reason why I only do pre-sales after 70% is to reduce the risk for players. Because all my games are solo made projects (with some helpers on debug and proof-read). If I had an accident, the project would definitely be abandoned. So pre-sales are actually selling 70% of the content that has been created.