The band, not the straps, provides most of the support in the bra. The band should fit securely but not tightly. You should be able to fit a finger between the bra band and your body. A new bra should fit best in the loosest row of hooks, because the bra will loosen with wear. You will switch to the tighter rows of hooks over the life of the bra. The band should not ride up. If there are gaps at the bottom of the cup, the band is too loose, a sign it is time to buy a new bra.
First discovered these bras years ago, and I wouldn't wear anything else. They are beautifully made, fit me perfectly, launder well and are comfortable. Love the ease of a front close and the wires never dig. I've always had a problem with straps slipping down until I found Strap-Mates. I have them on all of my bras and this handy, inconspicuous little item is truly a life-saver.