The short answer is that we don’t measure velocity anymore when using Trello.
We found that while Velocity is a very useful tool, it also has its downsides. On projects with irregular staffing, or at the beginning or end of projects its not as accurate as you’d like. It also sometimes becomes a number that clients focus on, which is not the point.
So instead we do our best to have open, honest communication, weekly planning meetings where we plan the week (at which everyone is present) and discuss what got done and what didn’t and why, and what we realistically expect to happen the coming week.
This isn’t perfect, it works well when things are going well, but can have trouble when things aren’t going so well. It also means that you can’t have automated enforcement of what is and isn’t expected to get done. But Velocity wasn’t perfect either, so for many projects we’ve chosen the overall tool over the specific feature of Velocity and it’s gone well.