By Upscale I mean steep a certain amount in 8oz of water (or better beer after you primary so you know how it will mix with the malt and hops). Start low and keep adding until you are satisfied with the flavor contribution.
Once you are satisfied with the ratio,and doing it by volume instead of weight.
.5 tsp per 8 oz was what you liked. So 5 gals is 640 oz or 80 * 8oz so you will need 40 tsp. The same philosophy works with all additions (This is how I get the right amount of spice and fruit in my beers)
And yes it is recommended to wait till after primary fermentation. The reason for this is that elderberrry flower is mainly added for its aroma just like dry hops and the CO2 generated during primary fermentation will drive much of that off.