School Assessment Model

I work in a school in the UK.  For our Year 7 to 9 pupils, they are assessed with what are called mastery levels as a current grade.  These range from Emerging to Mastered.  Pupils are assessed 3 times a year at the end of each term, which our system calls resultsets.  They also have a Target - which remains the same through each year/resultset.

We currently use Excel to track pupils - one worksheet per subject, which would look like:

English

Name     Mastery Level     Target        +/- Target

Pupil 1   Meeting              Meeting+   0.5

The mastery levels are turned to a number with a VLOOKUP to give us a +/- target.

We are starting to use Power BI - and have successfully looked at attendance and behaviour - but now the powers that be want to look at assessment and I'm struggling with it.

I have a table called Mastery Levels which contains pupil id, subject, resultset, mastery level.

Then, I have a table called Targets which contains pupil id, subject, target (a mastery level).

Both of these two tables are linked to what is essentially a lookup table - which contains the mastery levels and a corresponding numerical value (i.e. Emerging = 1,  Emerging+ = 1.5 etc. etc.)

I can produce a table which links to subject and class slicers to give me:

Name    Mastery Level   Target

Pupil 1   Meeting            Meeting+

However, I am really struggling to produce a +/- Target measure.

I've tried appending the Mastery Levels table with the Targets table so everything is in one table - and then used Merge Query to bring in the numerical value of each mastery level.

I've tried a few different measures, but no joy - the calculation is either wrong or in the table it doesn't react to the subject slicer on the page and a pupil will have all their subject mastery levels.  It's:

Target Difference = CALCULATE( AVERAGE(Mastery Levels[Points]),Results[Type]="Current") - CALCULATE( AVERAGE(Mastery Levels[Target Points]),Results[Type]="Target")

I don't know if I need to build in a RELATED bit so the Mastery Level for English (for example) finds the Target for English and then does the sum?

At the moment, we just have the autumn assessment mastery levels - before Easter we'll have a spring set - which adds to the complexity.

I'm sure there's more than one way to go about this - but if anyone has any thoughts as to a fairly simple way to go about this, I'd be extremely grateful!