I agree on most of the posts above, but I think I can simplify it a bit.

1. Dual rates are fine, but always use 100% of the available servo throw for your high rates. (this may be between 120-150%, depending on your radio)

2. Use as close to a 1:1 ratio mechanically. This means if your pushrod clevis is 1/2" from your servo center, put the control surface clevis 1/2" from the bottom of the control horn. This is not a hard and fast rule, but one extreme will give great strength with low deflection, the other will give high deflection with severely less holding power. The latter can cause flutter and destroy the airplane in flight. This is why you should never use EPA to reduce high rates!!!
3. Once your high rate is mechanically set, use your dual-rate adjustment to give you your desired low-rate setting.