When you are in veggitative growth you want the first letter N to be higher than the middle one.When you switch to flower you want the second letter P to be the highest.
As a example,i use general hydroponics maxi-grow and maxi-bloom.The grows numbers are 10-5-14,the bloom is 5-15-14.If you use a fert with too high a N number in bloom it will try to keep the plant from flowering.
You can get all of your questions answered if you look in the growfaq section at the top of the page.OPH