Nelson's ruby

[DaveyT]

Hello
I'm putting the Brewer's Gold to one side and focusing on my bag of Nelson Sauvin. I'm aiming to do 14l brew and split into two 7l FVs. One will be a blond Nelson-only brew while I want to make the other ruby red.
I'm thinking of only adding 20g of roast barley to the 7l which ought to have an OG of 1.042. I'll steep it very briefly then boil it for 15min in a pan before cooling and adding to the FV.

I do have a few questions:
Is this enough? If so, do I have a margin of error or will it turn into pitch if I'm a gram or two out?
What will the flavour be like? It's one thing chucking this in for colour, another if it's going to taste like I dropped a lit fag into the wort.
Would it worth using 25% crystal malt a la Sarah Hughes? If it ain't broken, and all that. I could also add some of this to the main batch, up things a bit and go for something Kipling-esque.
All thoughts welcome.

Thanks
David