I cant remember my exact MMR from last season but it was around 2200 (according to opgg). If this is true what you said about MMR calculation (My MMR from last season + 1200 divided by 2) then Master and Diamond divisions are worst for SOFT reset (i didnt calculate for challenger), because you lose much more MMR comparing with other divisions.
HIDDEN MMR DISTRIBUTION BY TIERS: CHART OF LEAGUES & DIVISIONS
Here we list estimate values of normal MMR for each ranked position. We call MMR normal when league points gain is approximately equal to league points lose. One division equals 60 MMR and league equals 300 MMR. Chart of average MMR values for each league and division
(THESE ARE APROXIMATE VALUES BUT THEY ARE VERY CLOSE TO THE REAL ONES):
BRONZE LEAGUE MMR VALUES: 0-1089 MMR
Bronze 5 MMR is 0-839.
Bronze 4 MMR is 840-899.
Bronze 3 MMR is 900-959.
Bronze 2 MMR is 960-1019.
Bronze 1 MMR is 1020-1089.
SILVER LEAGUE MMR VALUES: 1090-1409 MMR
Silver 5 MMR is 1090-1149.
Silver 4 MMR is 1150-1209.
Silver 3 MMR is 1210-1279.
Silver 2 MMR is 1280-1349.
Silver 1 MMR is 1350-1409.
GOLD LEAGUE MMR VALUES: 1410-1719 MMR
Gold 5 MMR is 1410-1479.
Gold 4 MMR is 1480-1539.
Gold 3 MMR is 1540-1599.
Gold 2 MMR is 1600-1659.
Gold 1 MMR is 1660-1719.
PLATINUM LEAGUE MMR VALUES: 1720-2019 MMR
Platinum 5 MMR is 1720-1779.
Platinum 4 MMR is 1780-1839.
Platinum 3 MMR is 1840-1899.
Platinum 2 MMR is 1900-1959.
Platinum 1 MMR is 1960-2019.
DIAMOND LEAGUE MMR VALUES: 2030-2330 MMR
Diamond 5 MMR is 2030-2099.
Diamond 4 MMR is 2100-2159.
Diamond 3 MMR is 2160-2229.
Diamond 2 MMR is 2230-2269.
Diamond 1 MMR is 2270-2329.
MASTER LEAGUE MMR VALUES: 2330-2500 MMR
Master league has no divisions you are going straight to challenger league upon promotion.
CHALLENGER LEAGUE MMR VALUES: ABOVE 2500 MMR
The highest possible league in game but players here have challenger points (analogue of elo system but only for challenger league
I made some calculations...
Master goes to Plat 3 (9 down)
Diamond 5 goes to Gold 2 (7 down)
Plat 5 goes to Gold 5 (5 down)
Gold 5 goes to Silver 3 (3 down)
Silver 5 goes to Bronze 1 (1 down)
P.S. I am not complaining, just want to know how it works!
Cheers