I'd say about 67% less effective than Redguard. That said, if you have high CP you could do almost 100% of Overland content as a solo Breton Stamblade.
Don't pull numbers out of your hat randomly, please.
Even without any CP, you could do 100% overland content as a stamina nightblade breton. That's basically my wife's character, and it's also her first MMO, and she hasn't had any problem whatsoever. Sure, in her early days she had no idea what she was doing and messed up alot, but it was because of her being a newbie, not being a stamblade breton.
It's absolutely not an optimised choice, but so long as you don't expect to be top in any "competitive gameplay", then you'll be fine, OP.