GW's Gale's Festival Mild - No Black Malt

[PaulStat]

Hello folks,

Doing this one tomorrow, one problem I don't have any Black Malt, I was going to use Roasted Barley instead, but it makes up roughly 5.5% of the recipe, so I suspect this might be too much. So I guess this leaves me with two options, do I

(a) Replace the black malt with the roasted barley and have it at just 4% of the malt in the recipe and increase the quantity of crystal malt
(b) Replace the black malt with chocolate malt, if I do this should I stick to similar quantities that the black malt was set at?

Thanks,
Paul