Maximum grade points achievable by training - 2000.0, or 34%. It is possible to get more grade points by taking damage to armour\modules.
The bonus is applied to to-hit chance only IIRC.
Crew training affects starting grade points, not maximal - 100.0 per level. Lvl1 crew gets 0% bonus (100 pts), lvl5 crew gets 12% bonus (500 pts). After that grade points can be increased by training by officer or taking damage.
If one runs out of trained crewmen, the crew will be filled with conscripts with no training whatsoever, and will get less points depending on amount of conscripts in the crew. If there are no crewmen in pool, the ship will get 0.0 point crew, or -10% grade bonus.