In the example you used you have 28,4% avoidance and 52% block combined 80,4% so you only have 22% to get a full hit, shield block gives only 22% block in that case so it should be 22/3=7,33% block from shieldblock. The rest looks solid
Did the math myself with 30% avoidance and 54% block used a bit different calculations but my conclusion was that with the blockmeta i took 0,77% more damage on average in the the 20 secs without shieldblock and 2,37% less damage while shieldblock was up. Making the blockmeta slightly better on avarage but i found the difference not big enough to switch yet since armor meta is more reliable and helps in situations with bad hit strings.
Blockmeta will get better when we get closer to 102,4% unhittable, since the armormeta is only better on the full hits we receive. Think I'll switch after 5% more block but also depends how i value EH, might be that when i start with hardmodes I'm not willing to sacrifice the EH the armor meta gives.