Grain substitution for Crystal 150?

Hello,
I'm currently boiling BrewPastor's Dark Night of the Soul, and considering my next brew (not for today), a mild that I saw in BeerAdvocate last year (not the current issue). The ingredients call for Crystal 150, but I can't seem to locate that anywhere. My two substitute choices are Simpson's Extra Dark Crystal (155-165) or Belgian Special 'B'. I have the Special 'B' on hand, but would have to order the Crystal. Can I substitute Special 'B' for Crystal 150, or should I try something else? Any other substitutions welcome.

Thanks

The Extra Dark is your best bet. Special "B" could be used if the quantity is under 4 ounces.

How about if the quantity IS 4 ounces?