If a 19 handicapper entered I guess they would play off 17, so a 10% reduction would be the first consideration ie they wouldn't play off 18?

If the limit is set to a Course Handicap of 18, then 17 would be the maximum Playing Handicap (at 95%).

If the limit is set to a Handicap Index of 18.0, then the course Slope Rating would determine what the maximum Course Handicap (18 x Slope/113) and Playing Handicap (0.95 x 18 x Slope/113) would be.