To expand on what Ulrik says, split the process into two parts:-
- Shooting - (have you managed to get a hit)
Including -1 for doubling; -1 for in cover etc. these are cumulative.
- Result - (or can you survive)
Use the best save available from cover or armour. These are not cumulative
Note,
1. MW hits can only be saved by Reinforced armour (so normal Marines would die, even in a trench).
2. TK hits are so powerfull that they cause multiple damage and cannot be saved at all by anything except titan shielding (TK shooting uses a slightly different process)