"Do you find that applying a percentage of your total system’s value of perhaps 5% or even 10% is a good metric?"
I'm going to go off the reservation a bit and say it makes me crazy when I read someone stating we should allocate a percentage of our overall system cost to something. The one I see most often is the rule of 10% should be spent on cables. It brings to mind the marketing genius of De Beers Diamond Company, all those years ago, selling the idea an engagement ring should cost 1 month's salary (it may be 2 now). Spending more based on some marketing idea, just seems silly. IMHO