With Standard apples, don't prune too much in the dormant season, concentrate on removing dead and diseased limbs, limbs that grow into the middle of the tree, and most of the strong upright 'suckers'. I would not take more than about 25 % in the winter. You want to do the balance in july or august after the spring flush of growth. Concentrate on some thinning cuts in the winter. Do not 'flat top the trees. leave a few shoots going straight up. I would generally strive to have about three main leaders or ideally a central leader but with Standard apple trees it is usually better to have three main leaders. I'd work more on removing the pendulous 'below 90 degree angle limbs in the winter, and deal with the upright suckers in July August. Just don't over do the pruning..........about 25% of canopy max.