While you have a great mindset of practicing transitions, I feel you are under utilizing practice partners, ladder will never be as effective as correct practice with teammates/friends.
For example, you want to know build A vs reaction A,B,C,D,E so you practice 5 games, then you want to do the transition out of A. so then you have, A vs, A1,A2,A3 ALL of the different possibilities. You can grind this out in a day. This is how it is done because its the most effective, sure it sounds boring etc, but that's how when you see Koreans make instant decisions, its because they don't have to think they already have played every possible situations and practiced what the best response is.
Try the practice partner way and i guarantee your game will be boosted 100x higher. Even if you don't have a team to do it practice partners can be effective for this as well.
The problem with this is that:
A) You need to have a practice partner;
B) You need a practice partner who understands the different builds well enough to help you;
C) You need a practice partner mechanically strong enough to execute those builds.
So, for example, you want to play a TvT. You want to practice build A. That person will have to know how:
A) Build A works;
B) Have Build A1, A2, A3 etc.
C) Know how those builds unfold into late or end game
D) Mechanically execute those builds on your level of play.
Damn that's asking a lot! :O This is only available in a pro setting, I'm afraid. Unfortunately, I have to agree with Unstable.
I recommend playing by time or schedule. Like what so:
A) Play 2 hrs every day of the week for 5 days, using the same build (impt).
B) Take 1-2 hrs categorising play into race divisions and analysing that specific build against race match ups and what you can work on when going up against different builds.
C) Refine build.
D) Play again until comfortable.
This takes forever. But you will be very good at one build. For example, I always 1raxFE. Now, after a year of SC2, I know two branches of 1raxFE really well - 1raxFE into marine/tank and 1raxFE into marauder/ghost/viking. I know what to do for all-ins rushes, late game, research timings, attack timings, expansion timings etc. little things like that. So while I might be boring, I thoroughly understand my build and what to transition to in late game. I must've played 100 games with 1raxFE, and I still have problems with Protoss. It takes a while if you want to improve alone.
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