May I suggest...
also leave a mouse afterwards (small line) to make it easier in the future
You almost have it right with the washer & magnet.
The problem is that washers don't weigh very much, and getting a line to drop cleanly down a mast without binding up us very challenging.
I've spent more hours in a bosun's chair fishing wires and rope than you would believe.
So, here is a surefire method.
Get an electrical fish tape which is at least the mast height.
In your case, a 50' fish is perfect. Home Depot sells these for about $20
On the end of the fish, attach 1 foot of bead chain.
Now, run the fish down the mast from the top.
You will know when it hits bottom, cause the guy at the base will hear it very clearly.
Now, insert a tubukar magnet, (home depot sells magnets at the end of a telescoping pen), and you'll pick up the bead chain instantly.
Pull the bead chain out of the hole, and attach a light line to it.
The man aloft then retracts the fish, and the tracer line is now at the masthead.
This is a no-fuss foolproof method.