I think if you ensure you've made life changing gains by...

I think if you ensure you've made life changing gains by selling down to your comfort level - the level of $$ you'd put into the company today - then you can ensure you don't miss out too much by selling a fixed $ amount each report or year. Doing that would mean that if you're wrong to sell and it keeps rocketing, your fixed $ becomes a smaller and smaller % of you holdings. Conversely, if it starts going poorly then you'll be selling a higher % and exiting faster and protecting your wealth.

For me, I want to have at least $1m in APT for the next year or two. I plan to trim as above at each report to get to my $1m comfort level. However, certain events would stop all trimming - eBay USA, Amazon, or Walmart.

Afterpay is only really just getting started in USA and is at 2018 stage there....

that's just my plan. everyone should get their own professional financial, legal, and accounting advice before doing anything.