Linking table values

Hi, 

I would like to report the performance of employees against a number of criteria. The data source is Excel table called Performance, example below:

I would like to be able to show:

• Visuals showing percentage of all employees at each rating (Below, Average etc) for each criteria. I hope to slice by 'Criteria'.
• For each employee, a visual of their performance across all the criteria

The assessment is not numeric however I know that to generate visuals that have the categories in order bad>good I will need to assign each rating a corresponding numeric index value. 

I have added a table called Rating which I would like to link to, to assign the value to each rating. (I've chosen -1 to 3 but I suppose that could be just 1-5?)

But I cannot work out how to link the value in RatingIndex to each entry in the performance table. I have looked at the RELATED function and LOOKUPVALUE but both need the tables to be related (I think) and there is no relationship between my tables. 
It feels like I should be able to do this in PBI without adding new columns into the source excel sheet to add a number next to each text rating.

When I report my Performance table I want the tool to 'see' and use the RatingIndex value rather than the text 'Below, Average etc'.
I hope that makes sense!!