very true. I have one screenplay in progress. I still haven't decided how it will start or how the story will progress.. but I know how it ends.

That's, I think, the advantage of fully outlining with at least 8 sequences (I usually go more granular - two page sequences).

I usually do either story idea or character idea, then the best ending and that gives you your Act 2 midpoint. Most of the time.

The midpoint is usually the worst thing that will happen or the best. Sometimes I try to do both.

I've tried a few times to start a script without really perfectly knowing the ending and it was an absolute mess at the end. So I couldn't more agree with you.
Know your ending or you're gonna be in big trouble...

Of course you know Casablanca:
• Howard Koch: “When we began, we didn't have a finished script...Ingrid Bergman came to me and said, 'Which man should I love more...?' I said to her, 'I don't know... play them both evenly.' You see we didn't have an ending, so we didn't know what was going to happen!” –Hollywood Hotline, May 1995

• Howard Koch: “The ending of the film was in the air until the very end... I was working every day on the set... I think we never really had the ending for sure... We thought of many possibilities and finally decided on the one that was in the film. That has proven to be the ending that the audience accepts.” –Hollywood Hotline, May 1995

• Julius Epstein: “Warner had 75 writers under contract, and 75 of them tried to figure out an ending!” –Hollywood Hotline, May 1995

I'm a little late to this but wanted to join in & say this is GREAT advice.

I don't know how many projects I abandoned before this tidbit really sunk in.

90% of the time you're deluding yourself with, "I'll figure it out along the way"... or, "It'll come to me eventually"...

I got burned too many times. Now I figure out the ending IN DETAIL at the outlining stage.

it's impossible to start without one