I've just made some tweaks to it, including adding IBKR Fixed, and moving the buy freq up into a comparison table. I squinted at the annualized effective cost calculation and I
think it works for 5-month frequency (for the IWDA/ES3/IWDA/ES3/A35 type of 5-month cycle that ST sometimes recommends), but I'm thoroughly uncaffeinated right now so I'm not sure why I thought it wouldn't work two months ago.
Here is the link again for convenience:
https://docs.google.com/spreadsheets/d/1BqIhbYNJF7VJbtj6PS08Csbyx1s4vs_03Kvh0pPRUCI/edit?usp=sharing
I think there are always intangibles that we can't address so easily though, because different folks value them very differently -- some folks really value CDP vs custodian brokers, some folks ignore custody and dividend fees, some are more comfortable with new robo-type platforms, some prefer not fiddling with RSP settings with specific deadlines every month...
Like I consistently have problems submitting SSB orders on time thanks to their silly operating hours not intersecting with my sit-down-at-non-work-computer time, so if I used RSPs I'd want to set it and forget it instead of fiddling with it on a calendar schedule. Others might legitly be ok with logging in to i-banking on the 12th of every month or whatever and changing a value, and that's good for them, but I think I'll lose a few dollars to save me the stress of timing things.
This confuses me a little -- "OR you're doing > $1,000 a clip" seems to suggest that if you're allocating $400/mth to IWDA, and you batch it up into quarters to $1200/quarter, then you should use IBKR? For that case, SCB would be around $80/year (4x USD 10.70 + forex spread on $4800) while IBKR would be $160/year (12x USD 10), so it's a bit odd.
It seems like the crossover point should be closer to $1000/mth
into IWDA, not $1000 per transaction or $1000 per month? I don't have the 3rd ed. of the book to check but the 2nd ed. seems to refer to $1000 per month, at least based on my reading of it.
That aside, this rule of thumb also leaves it wide open for the follow-up FAQ -- when should you buy less frequently than every month? What if we inverted the framing: if your monthly IWDA allocation is $1000 or more, buy IWDA every month through IBKR; otherwise, buy IWDA less frequently through SC, frequency decided by whatever gets the cost below x%.
That might squash at least one FAQ, so some people will ask the next FAQ about chionging to IBKR's USD 100k, but at least then that's a more interesting asset allocation / risk question than a fees question...