The calculation to get the difficulty is the same in LTC and BTC, there is only one difference, LTC diff is calculated to get a block every 2,5 minutes (150 seconds) and BTC is for 10 minutes (600 seconds).
To get diff : Get average Network hashrate of the last 2016 blocks (aprox):
BTC : 23644.89936 Gh/s --> 23,644,899,360,000 h/s
LTC : 598.55 Mh/s --> 598,550,000 h/s
Diff =(hashrate/s * avg_time_to_get_a_block)/2^32
BTC_Diff= (23,644,899,360,000*600)/4,294,967,296=3303154.282
LTC_Dif = (598,550,000*150)/4,294,967,296=20.90411726
So the fact that GPU get +- same hash but in Kilo instead of Megas doesn't matter, it is only a effect of the algorithm choosen, the value taken is hashes per second in both cases.
More clear if anytime LTC network hashrate would be the same than BTC, the LTC diff will be 1/4 of the BTC diff.
Regards
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