Tech improvement questions

[TheVen]

I'm looking for advice on how to calculate combat possibilities considering tech improvements. I am unsure how much of an advantage/disadvantage I have when my unit type is at a different tech level than opposing unit types. Some tech improvements seem to give more modifiers or whatever, but some others seem to make a unit virtually impossible to hit. Examples of the latter:

Sub and/or ship units and their evasion. Is a sub that has evasion of 3 impervious to op fire from artillery with sub attack of 1 or 2? How about other naval units? Also, is a sub/ship with evasion of 3 able to be attacked by aircraft with sub/ship attack of 2? Is it impossible, unrealistic, or just difficult? Do numbers really matter that much when attacking at a tech disadvantage, meaning attacking 2 subs with 2 aircraft or 4 aircraft? Or are you just exposing more aircraft recklessly?

AA air attack versus air power evasion. Similar to the questio above about naval evasion. If an enemy AA unit has air attack value that is 1 level higher than your aircraft evasion, how hazardous is this really? And conversely, if an enemy has bomber evasion that is 1 level higher than your AA air attack, should you scramble to match it?

I have been learning through pbem for sure, and I've tried to consult the manual on these things, but I've had some disappointing and unexpected results when in these situations and I am making the assumption that the tech values are the reason why, even if there is a difference of only 1 level. Any advice is appreciated.