My upstairs toilet is leaking, & I'm trying to diagnose the problem. I know that the tank valve needs replacing (we have to turn the water off once we flush & allow it to fill back up-which is a royal pain!), but the water supply line valve is leaking-1st, we had to turn the supply line valve off & turn really tight to keep from leaking onto floor, but now we have to turn it all the way on/to the left tight to keep from leaking. Once we replace the tank valve, will this help the supply line valve from leaking?

If I understand you here, I would replace the shutoff valve AND the supply line as it is a very inexpensive part.

Mark