Not sure if this is the sort of answer you are looking for, but this link was helpful to me.
2.5-3 ounces/5 gallons is considered "new barrel" extraction rates; using the other conversions on the page, "new barrel" extraction rates are achieved with 18-21 cubes per gallon.
Having paid the price by over-oaking once in the past, my 2015 elderberry got 14 cubes medium toast American Oak per gallon for about 8 weeks. I added them straight, no rinsing or boiling.
I wait at least until the secondary before adding cubes. If I add anything in the primary it would oak powder but I have never added powder to elderberry.
Wait till you rack and lose most of the sediment, dose with k-meta and put your oak in. Keep it in for at least 3 or 4 months. Rack again and more k-meta. Your wine needs to age for a few years to bring out all the flavors.
Thanks for the info, I know it takes a long time for elderberry to mature, at least 1 year. Not happy with that but, maybe it will be worth it in the end results.