<p>I want to get my Ender 5 plus to print at 300 °C. As such, I've edited the firmware and increased the <code>HEATER_0_MAXTEMP</code> to 315 °C.</p>
<p>In my slicer, I can slice and print at 300 °C, however, I cannot manually adjust the temperature on the LCD screen past the stock setting of 260 °C.</p>
<p>Any help in getting the manual adjustment fixed would be greatly appreciated.</p>

Your math looks correct, and is also a good approximation for what I've seen in the first few weeks with my own Ender 3.
Another way to calculate (to check yourself) is to calculate the volume extruded (nozzle area times extrusion percentage times print speed -- be sure you convert everything to the same units!) in a given second, multiply by the density of your filament (common PLA runs about 1.2 g/cm^3), and get a rough figure for how long it takes to print a kilogram of filament.  Your actual print time will always be higher than this approximation, because there are moves during which the extruder isn't running, infill is often set to lower extrusion level, and of course there's setup and cleanup time to account for.