We're all aware of the recently aborted [SpaceX Demo-2 launch][1]...
While talking with friends we discussed the venting of LOX pre-launch.

I presume that the reason for this venting is largely to achieve (or maintain) a "_full_" tank of Liquid Oxygen. In part, I presume it's likely easier to permit some to boil off and replace it with additional cooled LOX, rather than try to cool and pressurise a closed tank "_first time_" - which blatantly isn't going to happen within the design's constraints.

Is this even a somewhat correct presumption?

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Either way, the fact that _some_ LOX is vented is undeniable.

Do we know how much is pumped beyond the [~287,430 kg][2] capacity of the core and booster's tanks? or in other words: "_How much is wasted?_"

I appreciate that this figure will likely be significantly different to other models (for example, those that don't use sub-cooled LOX), and I'd be happy with any an answer, not necessarily specifically related to this launch.

During the conversation, I toyed with a ~130% figure (i.e: ~30% wasted), but honestly I pulled that out of thin air.
If this isn't something that we know (perhaps it isn't public information), then I'd appreciate an informed estimate.

  [1]: https://en.wikipedia.org/wiki/Crew_Dragon_Demo-2#Launch_attempts
  [2]: https://space.stackexchange.com/a/32732/36064