The math is correct. 12900 (fee)/257 (size) = 50.19 satoshi per byte or 0.0005
BTC/KB.
But 50 satoshi per byte is not enough to get a fast confirmation. I've seen a tx with 50 satoshi per byte not confirming since yesterday. The recommended fee is more than double that amount according to
https://bitcoinfees.21.co/.
You can use
https://www.viabtc.com/tools/txaccelerator/ to make a low fee transaction with at least 0.0001/KB confirm within few hours. I already submitted it but that link might be useful for future uses.