Question

how did you get (that's 0.17M x 1/2 = 0.085M)

Answer 1

mols pip = M x L = 0.17 x 0.250 = 0.0425
So it will take 0.0425 mols HNO3 to titrate it. How many mL HNO3 will that be? It will be M = mols/L
0.17 = 0.0425/L and L = 0.250
mols salt = 0.0425. Total volume = 0.500
M salt = 0.0425/0.5 = 0.085M
I've done these about a million times, I guess, and I know that when the SAME concn is used for AND base it is ALWAYS 1/2 of the molarity. It only works that way when the concns are the same, for different concentrations it is some other factor and not 1/2.

Answer 2

Just a minor correction about the 1/2. That is ALWAYS 1/2 of the molarity IF THE CONCN IS THE SAME and IF THE ACID AND BASE HAVE A 1:1 RATIO. That is, for HCl + NaOH (both monoprotic) or
H2SO4 + Ca(OH)2 (both diprotic) etc.