I'm a little confused. So the total lag = input lag + response time?
No one here has suggested a formula for calculating input delay. All that's been said is that the more stuff a monitor does to an image before its displayed, the more time between when you hit a button, to when it happens on screen.
Also, what should be used for MB in such TV monitors, HDMI or VGA? I have a big LG on my room, it lags like hell on VGA, but lags even more on HDMI. However, I've heard that is a problem of the big LGs.
There is no correct answer for this. From my experience, monitors that do well playing melty work well with whatever input you want to use. There may be cases where one works better than the others, but on monitors that already work well with melty, the differences are usually too small to notice. I have used VGA, HDMI, DVI, RCA, and RGB connections on a single monitor for running melty, and had them all work about the same. I've even converted between these and had it work about the same.
My general opinion:
Don't play melty on something that is sold as a TV. Play melty on something that is sold as a computer monitor. This seems like a good starting point. While I have seen some TVs that do well with melty, most all I've tried had huge input delay. Most computer monitors I've tried have pretty much none, or small input delay. What I do if I'm not sure about a monitor, when I can't find reviews, or don't feel like doing research, is I go to a store with my laptop and stick, and ask to hook up to monitors that are on display. When I find one that feels right, I buy it. Sometimes I take a buddy with me to see what he thinks too.