Understanding the gap of 1 , 2 , 3 of an 8

SRGM PDNS
, RGM PDNS
, , GM PDNS
, , , M PDNS
SNDP MGRS
, NDP MGRS
, , DP MGRS
, , , P MGRS
At each speed play atleast 5 mins non stop & try to Practice Upto 100 bpm if possible upto 120 bpm