I made a very nice Frontenac port style wine last year. I fermented the grapes then used a Pearson square to figure out additions of sugar and grain alcohol to make the percentages of the port style. Step feeding is better for flavor reasons I think, and I just couldn't figure out beginning sugar levels to hit a percentage of alcohol and residual sweetness. Fermenting dry and then making additions worked better for me. Frontenac really has a nice flavor to it.