yeah, you subtract but it still doesn't make sense in practice. it would be like saying second day of the week is Monday, i want to know what is the first day of the week so i subtract 1 from Monday. it obviously works but you should already know it is Sunday.

if you are coding for example in c# you hard code an enum that contains the names and their values like this:
enum Bip44Indices : uint
{
    BTC = 0x80000000,
    TestNet = 0x80000001,
    LTC = 0x80000002,
    ....
    BCH = 0x80000091,
    ....
}
then you use that wherever in your code you want.