I will recommend three possible solutions to your problem:

1) Shoot in Manual mode using an external light meter to obtain the correct settings.

2) Use the camera's internal light meter with the lens wide open to obtain the correct settings. When when you stop down the lens, do the mental math, or use a chart, to set the correct shutter speed in Manual mode.

3) Add an advanced Programmable AF Confirm Chip to the lens mount so that the camera will communicate with the lens. You can stop down the lens and then the camera in Av mode or manual mode will be able to properly communicate with the lens.

I use these chips with my manual focus lenses mainly because I can program the focal length and aperture into the chip so that it shows up in the EXIF data. With a DSLR I always shoot in manual mode and review the image to get the right exposure. Theoretically any of these programmable chips should allow more accurate metering. I have experimented in Av mode with these 3 Programmable chips but I have only found one that seems to give consistently accurate results.

The Adplo AF chip can be programmed to report any focal length and aperture. it also allows you to adjust the camera aperture using the aperture wheel on the camera. In Av mode I set the camera aperture to match the lens's wide open aperture. As I stop the the lens, the camera seems to know what shutter speed to select for accurate exposure.

In my experience the Big_Is AF chip (factory default programmed to 50mm f/1.4) allows you to change the camera aperture but does not seem to communicate with the lens for accurate exposure in Av mode.

The Tagotech "Dandelion" or "Euro" AF Confirm chip can be programmed to any focal length or aperture but does not allow you to set the aperture in the camera. This chip also does not seem to give consistently accurate exposure in Av mode.