Besides the fact that 80% crit chance (unless some specific abilities) are a bit over the top, it is even worse:

Let's stay with 40% shield chance and 40% crit chance (more reasonable values). Crit happens regardless of shielded or non shielded attacks. Then you have 40% chance to crit. On the remaining 60% you have 40% chance to shield the attack. That is 40% * 60% = 24% shield chance and 36% normal hit.

If the effects can apply all the time you would have 40% damage reduce and then of the remaining damage another 25% (25% *60% = 15%) so a sum of 55% now and of the remaining 45% you heal 15% (45% * 15% = 6.75%) so you're effective mitigation would be 40% + 15% + 6.75% = 62.75%.

With your logic there should be 3 rolls

1. Hit - Miss

2. Crit - Else

3. If else Normal - Shield

But it just simplier

1. Hit - Miss

2.

Simple table

100 sided dice

1-40 crit

41-59 normal

60-100 shield