I gotta agree with Archie on this one. I have my ailerons set for a really hot roll rate with probably -40% - -50% expo, and just fly what feels like a good rate for the roll based on the conditions and where the roll was started. Stick movement for me on the slow roll is probably anywhere between 10% and 20% of the available stick travel, it just depends. The key to not affecting roll rate when applying elevator is to fly the maneuver properly and having a well-trimmed airplane. If you use sufficient rudder while rolling to inverted, all the elevator will have to do is maintain the straight line rather than trying to "catch" the airplane before the nose drops. This way you're just barely bumping the down elevator before the other rudder takes over the job of keeping the nose up. Blend all those controls smoothly and you've got yourself a nice slow roll with a consistent roll rate.