What percent of leaders assessed should be rated "high-potential"?  Is there a percent range you try to stay in to insure you are being discerning enough?

If we are just talking about leadership level high potential candidates, there shouldn't really be a % because the pool is relatively small and these candidates usually are assessed to verify if they are suitable for C-suite roles.  Rather than going after a percentage, it will be more practical to assess if there are gaps whether those gaps can be bridged in the near to medium terms.  You will still consider them if a solution is identified.  For the rest of the population, ie junior to middle levels talents in the pipeline, we are aiming at 5% min.   

we're just getting into high-potential. 0-5%

Personally, I'm a fan of the 10 - 20% range. You want it elite, but not so elite you have too few people in each vertical to serve (especially if lateral mobility isn't quite feasible).

100% of your employees are high potential.  The question is how well does the organization's design allow people to live up to that potential?  

I know this isn't the answer you were looking for.  My intent is to highlight the strong influence that the situation someone is in has on their performance.   

I've seen low performers in highly controlling situations become rockstars when they were given the freedom to try their ideas, fail, and try again.    

The Corporate Leadership Council (now Gartner) used to have an estimate of the proportion of HIPOs you'd find across organisations - I think it was 15%. HIPOs contribute significantly more than 'average' employees, so I always see them as 'outstanding' or 'standing out' from the crowd, so I think the proportion should be lower rather than higher. I worked at a multi-corporate where we aimed at 20%. My current employer has just gone through it's first time with new talent criteria and the result is closer to 30%. One of our criteria is learning agility and we set 'moderate' as the benchmark; I'd say next time around we might want to set it at 'high' to reduce the proportion. Keep in mind that performance does not equate to potential, so you're looking to identify those who both perform and have capacity for roles with greater accountability and complexity in the future.