I come up with following odds for blocking 3 digits:
7/10 * 6/9 * 5/8 = 210/720 = .291666666 or 1 in 3.42857 like you came up with.
Since Stag draws occur 72% of time you get .291666666 * .72 = .21 or 1 in 4.76 if truly playing randomly by selecting random digits to block and randomly choosing when to play.
A person that is good at selecting digits to block or good at determining when to play stag can do better than 1 in 4.76 but how much better will be upto the individual. I think probably the most important filter to master would be TR to start with.
From my experience with DMP-BG I found the most predictable filters are ones that have 50% chance of hitting or coin flips. So other things to track would be Odd-Even by position, High-Low by position, Outer-Inner by position, Flip Digits by position. These are true coin flip decisions. So you can track these and block/play digits by position.
If playing Pick4 I think I like playing Doubles (12-way) in box form. There are 715 combos in Pick4 in box form of which 360 of the 715 are one repeating doubles. This is basically 50%. For Missouri which pays $500 for a double if you got good at knowing when to play drag you could play all 360 and profit $140 if you had the balls to play that strategy. Playing one-repeating doubles in boxed form is the advantage bet for Pick4. I know everyone wants to win $5000 but I think this bet is more predictable being that its a coin flip and should be predictable as to when to jump in and play drag. This is also a better bet than any Pick3 bets. You can think of the Pick4 one-repeating double like playing Pick3 single because you only have 3 unique digits but the bet is more predictable.
Anyway you asked so those are my thoughts.
Jimmy