I stepped over this while trying to solve a similiar question: "how much tx fees do I need to prepare?"... Though this is an older question, I thought it would be good still be ok to reply, so that others might benefit from the same research.
... Signatures are either 73, 72, or 71 bytes long, with probabilities approximately 25%, 50% and 25% respectively, although sizes even smaller than that are possible with exponentially decreasing probability.
Behind the Sigs there is normally the pubkey in hex, adding 33 Bytes for compressed, and 65 Bytes for uncompressed keys. So I calculate for a standard TX roughly 100 Bytes per sig.
You already have your figures for inputs and outputs. My std 2 inputs/2 outputs TX is roughly 400Bytes.
The Inputs part of a TX depends on the number of