I'm constantly testing older SLRs for possible inclusion in The Camera Shop, and sometimes their built-in light meter decides to quit for the day (or forever!). But I have a handy workaround for those situations.
I use an app on my iPhone called ProCamera HD that allows me to set the shutter speed and ISO. The process is simple. I set ProCamera to the same ISO of the film that's in my analog camera. I move the shutter speed scale until the exposure looks right on the LCD. Since I know the aperture is f/2.2 for my iPhone 6S, all I have to do is make the adjustment in my head to set the camera.
To do this, keep in mind the exposure triangle: one full shutter speed = one full f/stop = one full ISO setting. So if ProCamera is telling me that the right exposure is 1/500th at f/2.2 (I'm figuring 2.0 in my brain), then I can set my camera to that, or 1/250th at f/2.8 or 1/125th at f/4.
These are approximate of course, but with negative film there's enough latitude to cover me. And if I'm nervous about it, then I can bracket a stop on either side.
I've used this technique with many old cameras, and it's never let me down. And it's cool knowing that you always have an emergency light meter in your pocket.