This section is from the book "The Gardener V2", by William Thomson. Also available from Amazon: The New Organic Grower: A Master's Manual of Tools and Techniques for the Home and Market Gardener.
Your young Vines appear to have made good progress in the time you name, and if you have no special reasons for cropping them, we would advise you to cut them down within 2 feet of the front lights, and take only a bunch or two from each at the most. This will be the most satisfactory course in the end. Many young Viues are crushed in their youth by cropping them too greedily. We should prefer inarching the Madresfield Court on the Lady Downes rather than on Black Alicant. Your estimate of Madresfield Court is no doubt correct. White Lady Downes has all the good qualities of Black Lady Downes, as a robust free cropper and good keeper, and colours like a well-ripened Muscat.