I started with setting up variables for your numeric input, then, setups a Parse JSON with a prewritten table showing 26 numbers to letters.

[{"Number": 1,"Letter": "a"},
{"Number": 2,"Letter": "b"},
{"Number": 3,"Letter": "c"},
{"Number": 4,"Letter": "d"},
{"Number": 5,"Letter": "e"},
{"Number": 6,"Letter": "f"},
{"Number": 7,"Letter": "g"},
{"Number": 8,"Letter": "h"},
{"Number": 9,"Letter": "i"},
{"Number": 10,"Letter": "j"},
{"Number": 11,"Letter": "k"},
{"Number": 12,"Letter": "l"},
{"Number": 13,"Letter": "m"},
{"Number": 14,"Letter": "n"},
{"Number": 15,"Letter": "o"},
{"Number": 16,"Letter": "p"},
{"Number": 17,"Letter": "q"},
{"Number": 18,"Letter": "r"},
{"Number": 19,"Letter": "s"},
{"Number": 20,"Letter": "t"},
{"Number": 21,"Letter": "u"},
{"Number": 22,"Letter": "v"},
{"Number": 23,"Letter": "w"},
{"Number": 24,"Letter": "x"},
{"Number": 25,"Letter": "y"},
{"Number": 26,"Letter": "z"}]
Then find how what leader number you have with this expression in an integer variable I named leading
div(triggerBody()['number'],26)
then a condition if Leading is greater than 0
in the no branch, its just a filter array

if its greater than 0, meaning there is a leading number, expression for the base letter filter array is
sub(variables('initialinteger'),mul(variables('Leading'),26))
