In my opinion you will still lose some freon when changing the valve with the tool. There is one model out there the manufacturer claims has no loss.

I want to try a modification of mine to keep the valve insertion shaft tighter. Anyway without the tool you will lose probably almost all the freon.

Wear good gloves and good eye protection for sure when doing this.

Alternativly you may be able to buy a brass cap with a good o ring or rubber insert in it. Not absolutly foolproof but will not allow leakage on the threads. Perhaps not the ideal solution but should work okay.