A 1 KiB file will be effectively mirrored by OneFS (any file <= 128 KiB will be).   The OneFS block size is 8 KiB.  At N+3, you need 4 copies of the data to satisfy the protection policy.  So, the 1 KiB of raw data will consume a full 8 KiB OneFS block, and that block will be mirrored 3 additional times to satisfy your protection policy.  So, that's 4 8 KiB blocks, or 32 KiB total for the data, plus 2 KiB for the inodes, or 34 KiB for each file.  So yes, you have it correct...
1 billion of 1KB file, N+3 protection on NL nodes, how much space consume? Can you please let me know if I use the right way to get answer?  My calculation: 1 x 8KB x 4 + (512K x 4) = 34KB each file  ----> 34 TB for 1 billion files
space calculation