Instead of quoting the hypothesized formula, actually plug it in.
Plat IV MMR: 1950
(1950 + 1200)/2 = 1575 (Gold IV)
Silver III MMR: 1340
(1340 +1200)/2 = 1270 (Silver IV)
Anyone whose actually done placements would tell you.... that's not how MMR works in placements. MMR in placements is based on the difference in skill between you and your opponents. If your Gold team is filled with Plat V's, while the other team's Gold team is Low Gold/High Silver, you will gain negligible MMR. If your Gold team is filled with Silvers and you win vs Plats, you can win 200 + MMR, because the MMR calculator expected you to lose.
Furthermore, your placements adjust your team compositions based on previous Placement performance. If you are starting at a Gold III MMR, and win 3 games in a row as your first three games, you will be then fighting Gold I's and Plat V's, which increases your chances of losing. if you lose those games, you lose 100+ MMR, and get thrown in significantly lower brackets.
I've played this game since Season 4, had 5 accounts, and on 4 of those I actually studied the placements. I won between 7 to 8 of 10 every season on the first placements, and got placed Gold V-II. In an old account New placement, I've won 10/10 and got placed down 2 -3 tiers from where I was, because the teams I was playing against were based on that "soft reset" and gave me no noticeable MMR. (Season 7 I originally placed Gold III, in the original placements, but then the second Season reset (because they felt matchmaking during the initial placement was putting too many Plats with lower elos) shot me from Plat V to Silver II, because I went 7/10 due to some int feeders.
Your MMR gain over the course of a normal season does not increase/decrease any 11/12 points unless you have a horrible win rate. It's based on your % record vs the difficulty level of your opponents. if you are a 51-53% win player, you will get far more than 11-12 MMR. More like 60-190, depending on your rank vs theirs .
it's more complicated than any generalization you're putting out.