This is how you call the function from another function:

Code:
void testFunction(void)
{
    char bread = 'b';
    char loaf = 'f';
    char manual = 'm';

    // code

    calc_baking_time(bread,loaf,manual);
}
Note that the variable names you feed to the function during the function call do not have to be the same as the variable names in the function declaration/definition.

Code:
void testFunction(void)
{
    char one = 'b';
    char two = 'f';
    char three = 'm';

    // code

    calc_baking_time(one,two,three);
}