I too have taken on the care and feeding of my grandfather's M3 and lens kit (which now also get used on an M8). The add-on light meter barely works and is prone to scratching the top of the camera. I started using a Sekonic, but it stopped working reliably after a drop on the living room floor. So, now I try to meter by eye, and after about 20 rolls I've become quite good at it. Here's the approach I use now:
Ultimate Exposure Computer
I just remember "sunny 16" and work from there. I still use this technique on the M8, using it's built in meter primarily to confirm I'm within reason.
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Geoff