backdoor flush and backdoor straight draw

i know that backdoor flush draws are 4.16% but say you have in the hole...6c 4c and the flop comes out 5c Ks Ah, i have a backdoor flush draw and a bacdoor straight draw, what is the chances of me just hitting the backdoor straight draw? and some help from the math experts, i am not sure but do you just add the 4.16% backdoor flush draw to whatever the bacdoor str8 draw percent is? i don't think you do because it is always a little bit off, i think it is kind of what you do, but not sure, thanks people.