
Sounds like there was something amiss that didn't show, so although it's sad, this was probably the right outcome.
I hope you don't get any more need topping up, but if you do, I generally give just one feed of colostrum replacer, and regular ewe milk replacer after that. The packaging tells you what size of feed of the colostrum you need to give a neonate.
When tubing, because the lamb has no chance to say, "Woah, I'm full!", you do need to do little and often, as over-filling the first stomach causes all sorts of issues. I generally don't give more than 100-120ml in one feed by tube - these are strapping great Texel lambs, I'd give less to one of my primitive types.