Preseason Ranked Games & MMR?

Hey everybody,

I started playing League Season 7 and got placed Bronze 2 after placements. I didn't really practice on focusing on one sole champion, so after months of playing ranked I ended season 7 in Bronze 4 unfortunately. However, season starts next week and I'm currently Bronze 2, hoping to get to at least Bronze 1/Silver 5 by the start of next season (have been focusing more on warding, helping in team fights, objectives, etc.). Also, on op.gg (don't know how reliable it is) it states my solo MMR is that of a Silver 5 player currently and that it's above average for my current division.

I'm really hoping that i can do 5/5 or 6/5 on my placements and get placed at least Bronze 3/2 and not 4/5 (since I finished last season Bronze 4). My question basically is, how much of an impact does the MMR right before season starts have on placements compared to the MMR at the end of season 7 (my Bronze 4 MMR back in November)?