Constant exposure to me means the highlights are held constant as the filter is changed and the shadows become progressively darker. This is important because in printing we expose for the highlights and change contrast for the shadows.
I find my highlight exposure by using a highlight test strip printer found in Ralph's book Way Beyond Monochrome. It produces a series of test strips of the same area of important highlight. Once the exposure is found for the highlight the contrast (filter) is changed to get the shadows where they need to be. Each time contrast is changed a new test strip is made as exposure usually does need changed unless it's a very small change. This is the method for single contrast printing.
I've moved more toward split grade printing as mentioned by Matt. This is a much easier way to get to a finished exposure time and dodging and burning is much more effective this way as local contrast can be changed by burning and dodging during only one or both filter exposures.