When you have added two-thirds of the butter, taste the sauce and add a little more or all of the reduction. Then add the remaining butter, little by little, as before. The sauce should not be too vinegary but the vinegar should just cut the oiliness of the butter. Add a splash of water if it’s a little thick. Season, cover and leave in a warm, not hot, place until needed. The sauce can be reheated over a bowl of hot but not simmering water and lightly whisked again, but this may not be necessary.