What value of nMinimumChainWork and powLimit should I choose for my altcoin? // The best chain should have at least this much work.
        consensus.nMinimumChainWork = uint256S("0x000000000000000000000000000000000000000000000005c13f99f6d0b1a908");


how to calculate this parameter, i little not understand. It's not block in blockchain, how find this value if altcoin exist end i update sources with exist blockchain end for new blockchain.

consensus.powLimit = uint256S("00000fffffffffffffffffffffffffffffffffffffffffffffffffffffffffff"); 


end about this value , how it convert to diff xxx for understand what diff is it. example diff 1, 20, 1000 etc...
how to calculate this parameter, i little not understand. It's not block in blockchain, how find this value if altcoin exist end i update sources with exist blockchain end for new blockchain.


Because this is a new chain, the minimum should be zero.


  end about this value , how it convert to diff xxx for understand what diff is it. example diff 1, 20, 1000 etc...


It is diff 1, by definition.