The game says:
You can launch an attack before you have a complete firing solution - but you may miss.
The definition "may miss" is too inexact and gives no understanding about how does hit probability depend on firing solution, if at all. Game wiki says nothing, and searching for "sunless sea how is accuracy calculated" and "sunless sea hit probability" didn't give anything useful.
Based on my observations, hit chance is actually higher when a firing solution is more complete, but I don't know the exact formula.