I was just wondering what to do when it will leak
Use both good PTFE tape and pipe dope. Less likely to leak. If you still get a leak, tighten more, using a longer wrench if you cannot tighten with the short wrench.
If using a flexible supply connector, rather than chromed solid brass, I think having the output of the right-angle stop valve point somewhere other than straight at the toilet, lets the line drape better. Pointing horizontally to the right would be good I think.
Your new valve and escutcheon will look a lot better. If the new escutcheon is smaller, use that paint you saved to paint the wall that was covered by the escutcheon.
I expect the OD of your nipple is either 0.840 (1/2 inch) or 0.675 (3/8 inch) inches. You will need to know that to select a valve and an escutcheon.