What stepping is the DSLR set to for metering? That 75-300mm zoom strikes me as oddly-fitted to the task for metering: the camera compensates for zoom extension. If the camera has multispot and average capability, use that rather than a single spot reading. What strikes me is how cumbersome this exercise appears. Invest in a hand-held meter.
As each exposure comes out through the trial, get prints done from it as "reference cards" to what you are doing.