Power loss during transmission
=> Short answer: Sounds reasonable. 

Common approach would be:
Try to approach this way: What causes the power loss if you transmit power along a line?

For simple, resistive loads (based on Ohms law: \$U = R \cdot I\$ and \$P = U \cdot I\$)


The current flowing through the line (which is the current that you may use as well to power a device connected at the end of the line):


$$P_{loss} = R_{line} \cdot I_{line}^2$$

or


The voltage drop along the line (from where you feed in the power [start] to where you extract it [usually end of the line] - NOT the voltage between line and ground, which I assume you 50kV are)


$$P_{loss} = \frac{V_{along\;line}^2} {R_{line}}$$

However, since you seem only to know the load and line-to-ground voltage, using

$$P = I_{line} \cdot V_{line,gnd}$$

to substitute the current is correct. The resistance appears reasonable for a longer medium voltage line and &lt; 1 % ohmic losses is not that bad.

If you are interested, take a look at this page:

AC Transmission Line Losses, by Curt Harting

For AC: Use RMS values. If you have a multiple phase system, make sure you dont confuse line-to-line and line-to-ground voltages... Also the skin effect has to be taken into account.

General advice: If you are referring to values per length, use a dash: [P'] = W/m and also use them if you write something like "P = ...".