if the resistance of the wire is known, what formula will tell me the resultant voltage?

With a

voltage divider, the voltage is divided in proportion to the resistance.

Or, since the current is the same in both resistors, you can calculate the current from the total resistance and total voltage, and then knowing the current through each resistor, you can calculate the voltage across each resistor.

If you connect a voltmeter to your circuit, the resistance of the voltmeter becomes the 2nd series resistor. The circuit is completed and a tiny amount of current flows. (The current is small because the resistance of the meter is high).

nope. The voltage varies depending on resistance.

Are you measuring a voltage drop with a voltmeter/multimeter?

There is no voltage drop until you connect your meter, (or until you connect

*something* to complete the circuit). because there is no current flow unitl you connect your meter. And, you will only measure a voltage drop if the resistance is very high.

Another example is that same battery and a 30ft wire. the voltage measured at the end of that 30ft wire will not be 12v because of the resistance of the wire.

Again, only if there is current. And in this case, your meter cannot measure the voltage drop, because the resistance is so low compared to the meter resistance.