I don't like defending from Western Ukraine, since they can just as easily send a blob to Bessarabia and expose Ukraine; then your Belarus troops are isolated from that front, and Ukraine will generally fall rather easily.

I prefer to defend from Bryansk, with a smaller group in Novgorod. Bryansk is 2 spaces from any territory on the front, and gives a LOT of flexibility in how you respond to Germany. From there, you can either split your army into aforementioned Belarus/West Ukraine, or send it just one way, if Germany doesn't split up its men.