I agree with
@foxy that's it's difficult when it's a user upper, though I have a slightly different opinion regarding the amber malt... I think you want a bit of roast but more restrained than stout...
If you want to know what you're shooting for, Fuller's London Porter is excellent
The Greg Hughes Brown Porter recipe is also very good, though not a precise clone of Fuller's.
In addition to base malt I use something like 7-8% dark crystal, brown malt and chocolate or chocolate rye malt (actually planning to experiment with a bit more, but that's by the by).
I don't see a major issue with your crystal malt, just be aware it might be a wee bit high if there's not enough roast to balance it.
I reckon the black malt is getting the colour where you want to be. I think you could actually ease off a bit in terms of colour but I'd leave it as it might bring a little bit of roast.
I use 10% amber malt in a pale ale and it's not roasty at all. Be aware amber (and brown) malts vary - I don't know which one I use or what you have. But personally I don't see a huge issue trying 20% amber as an experiment, of have thought it would bring a bit of toast, if not roast.
Hops - I use Northdown in my Porters, I like it. I think the GH recipe uses EKG. Never used Bramling Cross, but I'd certainly be interested to know how it goes in a Porter.
Have fun experimenting whatever you decide - at the end of the day it's a hobby not a Space Shuttle launch and you'll more then likely make beer of some kind!