Knowing two cells in a list - count the number of cells (levels) between them.

I have a list of alpha numeric grades.

1a 1b 1c 2a 2b 2c 3a 3b 3c 4a 4b 4c 5a 5b 5c

I know the start level and the final level.
I need to find out the number of levels between the grades

Start Finish Levels

2b 4c 7
4a 4a 0
4a 4c 2
3a 4c 5

I am thinking of assigning the alphanumeric with a numeric (1 to 15) then use lookup and then subtract the 2 lookups, but I am thinking there must be a more efficient way of achieving this? Counting the cells between the levels or something?

I have attached the .xls containing the above data.

Struggling with how to achieve this.

You can use Vlookup look at attachment