It is also important to note, and I have not personally seen results for other than flagship products (*istD, K10D, K7D and K5D) , that metering with manual lenses (non A lenses ) is very inconsistent as a function of aperture.
See the graph below
Also note that some bodies meter differently for M42 lenses in M vs Av mode,
So you should check your histogram regularly. Also note, A lenses can work in all modes, and in M work differently than non A lenses.
Specifically,
For M lenses the lens is stopped down and metering is done in stopped down mode to set the exposure time. You will notice that metering is only accurate between F4 and F5.6 because this is the native aperture range of the kit lens. For all other apertures it is off, and you need to think a little about this error.
for A lenses in M mode, the camera meters wide open, and compensates for the metering error for wide open aperture and sets shuttle speed appropriately. The lens is not stopped down to meter.
What I recommend is that you meter off a neutral subject, I use tree trunks, paved (moderate grey not black) roads, gravel roads etc, to get an exposure reading for the subject I am shooting, then leave exposure alone until conditions change.
Manual lenses are fun, but more time consuming, so slow down, learn how your lens behaves and then enjoy the results