Pseudo 20G gives me by far the most trouble in the TGM series, especially when it's almost 20 but not quite. I often overshoot "notches" on the stack because I think the piece is going to land there. So, what's the best way to get used to this?
Stack high, pyramid-like, and consider yourself in 20G. Do not rely on placement that would be possible in low G but isn't in 20G.
If in doubt, treat it as 20G and just tap your way around. That way gravity will definitely drop your piece straight down in between taps. If you know where a piece will land, hold left/right and use DAS as it was intended.
There are 1 best input combinaison for EACH placements for EACH tetriminos. By the time you will learn these moves. Today I continu to get fucked by the problem you mantion in this topic, at TAP MASTER 400 and TI MASTER 250+.
You have to memorize the speed of the pieces before Lvl.500 in order not to be surprised by the changes. Level Gravity 0 4 30 6 35 8 40 10 50 12 60 16 70 32 80 48 90 64 100 80 120 96 140 112 160 128 170 144 200 4 220 32 230 64 233 96 236 128 239 160 243 192 247 224 251 256 (1G) 300 512 (2G) 330 768 (3G) 360 1024 (4G) 400 1280 (5G) 420 1024 (4G) 450 768 (3G) 500 5120 (20G) From 420 to 500, you can see speed decreasing. Be aware of that when you play so you can adjust your brain to the gravity.
My only advice: keep playing from level 0. In the time it takes to get consistent at getting higher grades, you will get to know 1-5G inside and out. I actually find it the most comfortable part of the game nowadays, because there are so many things you can pull off that won't work in 20G.
I place this picture to show an example of your problem, I hope I am right, even if I think this example is not really good but it is in your context. If you have this situation in 20G (left field), you can just DAS to the left during IRS CCW and lock. At 400, if you are not sure of the instant you decide to charge the DAS, you can face trouble like the middle field in sending too far the J At this moment, the safer maneuver is : IRS CCW, wait the tetrimino appears and then just 1tap on the left and lock.
I haven't really bothered learning the intricacies of 2G, 3G, 4G, and 5G, nor the order they come and go. The basic idea is that speed builds up from 300-400, peaks at 400 for a bit, then comes back down as you enter the 20G threshold (at which point gravity maxes out, duh). What I tend to do is avoid placements that need double or triple tapping starting at 1G, and prepare to react quickly during the 300-500 stretch if you're DASing and a piece surprisingly doesn't make it. Otherwise, stack high and pretend it's 20G.