It really all depends on your suspension geometry as there is no definitive answer.
I think that Dave from Dampertech recommends 300lb all round but may be wrong. It was a long time since I spoke with him.
What is critical is to get them set up right. It's a bit like tuning an engine. The most cost effective, simple and best thing to do is to get the manifolds and block matched and get all tuned and working properly on a RR session.
The same is true with suspension. You can have the wrong configuration and make it work but it's always best to get the basics correct with some expert input.
As Robin Hoods tend to vary between specific models, let alone between different models or even converted suspension from Pillar, there is no "one size fits all" answer.
Personally, I am starting with a converted sliding pillar to double wishbones but mine are different from every Robin Hood out there because we took the decision that we were not going to mount on the top chassis. I have started off with 300lb but will not know till I get it out on track whether this is correct or needs changing. I'm also running a Zetec which is slightly lighter.
If you have a standard (ish) configuration, then go with what you have to a suspension specialist and see what they come up with. If you haven't got anything yet then 325lb isn't a bad place to start.