I recall a speed drill Scott Lee showed us several years ago. He had 4 speeds. All start with the cueball one diamond from the head rail.
1. The poof shot. Cueball to wind up within one diamond of the foot rail.
2. Lag speed. Self explanatory, end up at head rail.
3. Medium speed. Head rail to foot rail and back to within one diamond of head rail.
4. Hard speed. Head rail to foot rail to head rail and back to within one diamond of foot rail.
One I had the hardest time with was the poof shot.
Like sappo said you could add variations by having the cueball fall within one diamond either side of the center pockets.
I think that by learning stroke control as it applies to speed using only the cueball it removes variances that inconsistant hits on an object ball would cause. For instance, if I tutored under JohnB (God, I wish I could) and he used a method like this to teach me speed of hit, then when a shot came up and he told me to use #4 speed, I would know that if he had taught me that #4 speed made the CB travel 'X' lengths of the table I would, hopefully, be able to shoot the shot with the correct speed of hit that I had learned. Does this make any sense? Heck, I'm old and I confuse easy.
Later, Pel