Think finess instead of bump. If you want more walk, you already know that's a 4 beat gait, so each leg hits the ground at a different time. So if you want more walk, each time a hind leg is about to back down to the ground, that's when you want to finess your horse on that side. If you ride the walk and really concentrate on feeling the horse's movement, you will notice that the belly swings away to the other side to make room for the hind leg on that side to come forward. If you belly is on the left, the right hind is coming forward. So when you feel that belly start to get out of the way, that's when you apply the aid. I guess you could use both legs, but it doesn't make sense to me since you are only asking one leg at a time to do something.
You just have to see your distance...you don't have to like it.