You don't have to go available exactly when your block starts. You just have to be available before the 15 minute mark, and during the 15th minute.

You can watch your data arrows, at the top of your phone, and see how it goes a little bonkers during the 15th minute, while it takes attendance.

The safest way to drop a block, without getting penalized, is to go available at the 13th minute, drop block at the 16th minute, then go unavailable quickly.

It can be a stressful 3 minutes!! And it's possible to get an order during this time.

You DO get GH compensation for the full 15 minutes of the dropped block when you do it this way.

And you're not marked absent.

If you see the yellow, or orange? (I forget) banner warning about you're trying to drop a block that you've already started, something like that, then you have jumped the gun by a few seconds. Go back, try to drop it again in a few seconds and when it's the normal happy blue message just choose 'something else' as your reason and it should be fine.

They periodically change how blocks are dropped, but this way currently works for me.