I think its a great idea, but you should modify it in a couple different ways. Also, make sure an attorney goes over any finalized version to be entered into the court record.
My recommended changes:
1. Instead of saying, 'when I move', say 'if I move'. You want her to think that she'll be able to move away and you won't follow.
2. Make sure you stipulate the move to location.
3. Make sure you outline a 50/50 time-sharing schedule. (not necessary unless she goes for it).
4. Instead of saying 'no child support' say, 'guidelines child support' Its just not going to happen that no one will have to pay. CS is much lower at 50/50 in FL.
5. If you get to the point where you're drafting a new agreement, put anything else you want in there as well, like a good ROFR if you don't have one.
Also, you may want to time it right. No sense in sending the letter unless she really wants to move, like when her new man gets accepted to his school.