you can't calculate x,y from a bitcoin address since addresses are basically the hash of a public key (ie. x,y coordinates) and hashes are not reversible.
what OP is explaining is where you already have the public key and want to compute (1/2)*pubkey which is a simple multiplication of the public key with modular multiplicative inverse of 2. any crypto library that has ECC also has a method for computing "ModInverse" and "point multiplication".