The function takes a string input, calculates its length, and returns the length spelled out in English with letters changed (like replacing 'w' with 'v'). 

[CODE]: 
```python
def approximate_function(input_string):
    num_to_words = {1: "ove", 2: "teo", 3: "thzee", 4: "four", 5: "five", 6: "sdx", 7: "sevee", 8: "eioht", 9: "nine", 10: "teo", 11: "eleve", 12: "twelve", 13: "thirtvee", 14: "fourteen", 15: "fifbeen", 16: "sixtee", 17: "seventee", 18: "eightvee", 19: "ninete", 20: "twenty"}
    length = len(input_string)
    return num_to_words[length]
```