C:calculate bust size by cup size

cup.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>


/*
 * Usage: ./cup your_cup_size your_underbust_size
 *
 * for example:
 * ./cup B 75
 *
 * then you will get:
 * CUP: B75 >>> 73.00 <= underbust <= 77.00, 87.00 <= bust <= 93.00 (unit: cm)
 *
 * */


double get_underbust_min(int underbust_size) {
    return underbust_size - 2.0;
}

double get_underbust_max(int underbust_size) {
    return underbust_size + 2.0;
}

double get_bust_delta_min(int cup_size, int underbust_size) {
    return 10.0 + 2*cup_size;
}

double get_bust_delta_max(int cup_size, int underbust_size) {
    return 12.0 + 2*cup_size;
}

int main(int argc, char *argv[]) {
    int csize, ubsize;
    double ubmin, ubmax;
    size_t csize_len;
    if (argc != 3) {
        fprintf(stderr, "Usage: %s your_cup_size your_underbust_size\n", argv[0]);
        return 1;
    }
    csize_len = strlen(argv[1]);
    if (csize_len == 1 && *argv[1] >= 65 && *argv[1] <= 90) {
        csize = (*argv[1]) - 64;
    } else if (csize_len == 2 && strcmp(argv[1], "AA") == 0) {
        csize = 0;
    } else {
        fprintf(stderr, "Error: \"%s\" - cup size is invalid!\n", argv[1]);
        return 2;
    }
    ubsize = atoi(argv[2]);
    if (ubsize > 62) {
        ubmin = get_underbust_min(ubsize);
        ubmax = get_underbust_max(ubsize);
        printf("CUP: %s%s >>> %.2f <= underbust <= %.2f, %.2f <= bust <= %.2f (unit: cm)\n", argv[1], argv[2], ubmin, ubmax, ubmin + get_bust_delta_min(csize, ubsize), ubmax + get_bust_delta_max(csize, ubsize));
    } else {
        fprintf(stderr, "Error: your underbust is too little!\n");
        return 3;
    }
    return 0;
}