Clarify corresponding lotus conf var for deal-start-window

[f8-ptrk]

The README says:

You should set correctly your --deal-start-window value to be a bit larger than your miner node configuration, to cover the time required to download and import data. If this value is incorrectly set, your miner node will reject proposals because the DealStartEpoch attribute is too soon for you to accept.

it is not immediately clear what lotus config var this refers to. needs clarification.

[jsign]

I can add some extra comment there to say that it's related to ExpectedSealDuration, but probably isn't the only factor considered. Unfortunately, I haven't found a miner that could explain in detail how to do an exact calculation for them so they usually set it up 1.20x that value or similar.

In any case the semantics of this flag are clear: we want a value to set the DealStartEpoch in the proposal, and not get your miner to reject the deal. I'd guess every miner should be able to answer that question since it's part of their configuration and contract.

[f8-ptrk]

i think it is this StartEpochSealingBuffer var that is corresponding to that - thanks to consulting with @Reiers on slack

[jsign]

@f8-ptrk, if you come up with a compelling answer, feel free to drop it here so I can add it to the docs.
I'd prefer some storage provider to write this part correctly since probably they'll do better than me.

In any case, feel free to close this issue when you think is appropriate.

[f8-ptrk]

i am going at the thing right now and will see what i find out after running a a bit and then adjust if i think it makes sense - i was confused what exactly this was referring to on the lotus side