Regarding the security deposit/bond requirement, it is explained on the tezos official documentation:
Security deposits¶ The cost of a security deposit is
BLOCK_SECURITY_DEPOSIT = 512 XTZ per block created and
ENDORSEMENT_SECURITY_DEPOSIT = 64 XTZ per endorsement. Each delegate
key has an associated security deposit account. When a delegate bakes
or endorses a block the security deposit is automatically moved to the
deposit account where it is frozen for PRESERVED_CYCLES cycles, after
which it is automatically moved back to the baker’s main account.
Since deposits are locked for a period of PRESERVED_CYCLES one can
compute that at any given time, about ((BLOCK_SECURITY_DEPOSIT +
ENDORSEMENT_SECURITY_DEPOSIT * ENDORSERS_PER_BLOCK) *
(PRESERVED_CYCLES + 1) * BLOCKS_PER_CYCLE) / 763e6 = 8.25% of all
tokens should be held as security deposits. It also means that a
delegate should own over 8.25% of the amount of token delegated to
them in order to not miss out on creating any block.
This is where the “8.25%” number comes from. However it assumes a specific number of the total XTZ actively staking to be 763,000,000 which is not the actual situation.
You can also interpret this number as the security deposit of “the entire network” in some sense however this does not tell you accurately what is the capital requirement for an individual baker.
The actual number of xtz staking can be obtained in real time on tzstats here.
Currently the amount staking at height 313728 is 503,461,245.334 xtz.
Using that number the effective percentage of xtz that is needed as security deposit is roughly 12.5%.
Note also one more time that this number is just an expectation because the realized number depends directly on the number of rights that a baker is allocated from cycle to cycle and that number fluctuates randomly (with less variance as the number of rolls grows though) so the bond requirement fluctuates accordingly.