1. First I created new score fields to andbox test my new scoring system.
2. Afterwards I spend a few days retroactively determining the scores, as if the scoring was implemented a year ago.
3. Next I exported all leads of the past 12 months to excel (including opportunities)
4. Now you create buckets of lead scores, and check if opportunities exist in all the buckets.
You are aming for no opportunities in the lower buckets.
If so drill through to detect why the lead score is low, and what you could have done to improve it.
Also there should be no disqualified, blacklisted, employees in the higher lead scores.
If so drill through why they didn't receive any negative scores.