How to use AutoComplete in Succession Plan Candidates to default Readiness Code

Summary:

We need to use Auto-complete rules to default value based on a field.

Content (required):

The customer asked to get the value of Readiness automatically based on the candidate's overall fit; candidates with 90% or more to be considered automatically as (Ready Now) and below 90% to be considered automatically as (Not Ready Yet) - We searched in most Setup, using and Administration documents to find any related answers or ideas but we didn't get a solution for it, So we decided to use Auto-complete rules as a workaround but we couldn't get the overall in the rule.

so please advice!