Calculating Estimate Netspace

[wiryonolau]

How to calculate estimate netspace accurately

There is get_network_space API that have older_block_header_hash and newer_block_header_hash
for newer_block_header_hash I just use the block hash I want to check
for older_block_header_hash which hash should I use ? all the netspace result seems very different from actual ( shown on many explorer )

Is this not the correct approach ?