Wow, thanks for all the thoughtful responses.
Trouble is, as you said dungoofed, the theory says that lump sum beats dollar cost averaging on average - and in a way dollar cost averaging is a form of market timing. And yet, I do not want to be that outlier!
Why would the size of the lump sum vs. my overall portfolio make a difference to the plan? (~14%, as it happens)
I certainly am working and committed to saving a chunk every month (very approximately 1% of portfolio), though I have not at all committed to a plan of what to do with it. It's accumulating in a 4% cash account until I work it out (or buy chunks of various Vanguard ETFs at random times because I know they're going to be in the answer somewhere and I can rebalance later).
At 31 years old in a well paid career I enjoy, I don't see much merit in keeping any asset allocation in cash. I'm prepared to take the risk of working a couple more years (or going back to work for a couple of years after ER) in the case of terrible market performance in the late years of accumulation / early years of ER, as a tradeoff for the statistically more likely scenario of shares performing better than cash enabling earlier ER.
My most likely dollar cost averaging scenario would be to put half in now, half in 2-3 months (or earlier if something that looks like a market dip comes along). It's not a large enough sum to be worth the brokerage of splitting it into more chunks, I think, plus I will have future chunks coming in from saved working income anyway. However, every time I've done that so far, I've regretted it, because the market has gone up. Would this be the exception? Bah.
