How to calculate low/medium/high fee transaction fee? I have a website where the users can send and withdraw funds using Bitcoin, I'd like to give them the chance of set their own custom fee (Low, medium and high) instead of the site to choose a fixed fee or than allow bitcoind to choose it.. 

For it, after read and investigate, I've seen that bitcoind have a json-rpc call with the name of ''estimatefee'' which allow us to calculate in real time the fee for the transaction appears on the specified blocks.

I guess that for a high it should be checking the fee in the next 2 blocks, for a normal fee in the next 12 blocks and for a low fee in the next 25 blocks, I've saved the obtained results and I Have comparate them with the data provided by Blockcypher's API: curl https://api.blockcypher.com/v1/btc/main , which it give us a higher fee on the low priority than in the medium, this is weird for me, but except that the rest of the data provided are quite inferior to those that I Have obtained, which makes me think that maybe I am making the calculation of an incorrect way (as all the fees I have obtained are at least the double than the one provided by Blockcypher).

And here is the data obtained from bitcoind:

bitcoin-cli estimatefee 25
0.00198478

bitcoin-cli estimatefee 12
0.00265746

bitcoin-cli estimatefee 2
0.00352903


And here is the data from Blockcypher:

"high_fee_per_kb": 174625,
"medium_fee_per_kb": 83628,
"low_fee_per_kb": 90769,


I haven't found many information related with this, if someone could help me, I would be grateful.

Thanks in advance
Here is a post The Challenges of Bitcoin Transaction Fee Estimation discussing the determination of Bitcoin fees.