At what point does it become commercial to use the NS premium support? Historically i've internalised it due to the cost of the support vs me supporting internally (and the relatively low volume of support tickets we would have raised)

IMHO, the only reason you might go with premium support is so you can call in for support tickets. When you call, you can (usually) get someone on the zoom "right away" (after waiting on hold for 10-20 minutes). As annoying as that can be it, it saves you about 8-24 hours of waiting for the rep to email you back and then playing email tag to find a time to zoom. So for me it's really about better response/resolution time. Maybe just my personal bias, but not sure there is a point at which premium support is going to save you money. Used to be the case before the "authorized users" situation that you could point all your level 1 support questions (password resets, people with super basic questions) to them, but not anymore, so even if you had a ton of simple questions across a big user base, you're still going to need an internal employee to funnel them through anyway.