Ah ok, your confusion makes more sense to me now. You shoot in shutter priority mode, where you set your shutter speed and then get an aperture suggested after metering.
In the case of using a genuine FD mount lens it would be perfectly fine like you do. Meter wide open and then go to the suggested aperture.
However, your Helios or Biotar is almost certainly connected via an adapter, and in this case the camera has no way of knowing what aperture the lens is set to, or what its widest aperture is, in order to adjust its meter reading automatically.
Chances are that it suggest a totally wrong aperture because of incorrect assumptions inside the camera.
Possibly this is what the store owner was talking about when saying the internal meter cannot be used.
I've never used the AE-1 myself so I cannot tell you whether or how you can meter stopped down with that camera. Others might be able to help you out with that.
But if you own a smartphone, I'd suggest to use a smartphone light meter app and meter the scene with your phone. Then select those settings on your camera and take the shot.
This way you omit the camera's internal meter completely and should get fairly accurate exposures.
thank you again for your comment, I think we are getting somewhere to understand what is going on. Please don't leave me now :'-)
You said "the camera has no way of knowing what aperture the lens is set to, or what its widest aperture is".
I don't get this. Why does the camera need to know those information? If a shutter speed is set, and the ISO value so, reading the amout of light that is coming through the lens, the meter only has to tell the only missing information, which is the aperture. Where am I wrong?
Thank you again for your patiance!