How can you calibrate extrusion multiplier value in Slic3r WITHOUT changing printer settings in memory?

Most tutorials ask you to change the printer settings using the The command M92 E420 will set the new value which can be saved to memory (so that it is available after a printer power off/on cycle) using M500.

But I just want to modify the Extrusion Multiplier in Slic3r's settings. It's default at for each filament, and I'd like to modify this by adding a profile for each roll I have after calibration.

What's the best way to do this? If I measure extruding 100mm and checking how much is left, what is the formula used to get the new EM value?