Whay are you having to do this all the time?
I've always been doing this since 1E AD&D.
If you use the proper level monsters for the player's levels and they are equipped properly you shouldn't need to checking the maths for every encounter.
There appears to be an implicit assumption of the players having magic weapons of an appropriate level. I don't know if 4E was originally design this way, but in practice this assumption makes the combat encounters less "grinding". But on the other hand, this assumption does make the combat encounters somewhat "cookie cutter" regardless of level. In a previous recent thread,
http://www.enworld.org/forum/d-d-4th-edition-rules/270715-1-level-instead-level.html
it was shown that the to-hit roll scales approximately as:
~ d20 + level
for an attack using a player's primary stat along with the assumptions of magic weapon enhancement and regular player ability stat increases.