Most encounters should be of Average difficulty. One (or maybe two) in an adventure can be Weak and/or Hard. Only one encounter should ever be Deadly.
It is recommended that no more than 1/4 of the encounters be traps or puzzles (unless such a flavor is your goal for the adventure).
To create an Average combat, just make the total Defense values on each side roughly equal. (Heroes have Defense 6, so multiply by the number of heroes and keep this number handy all game.) A Weak encounter would be only half the total of party Defense, while about 1 1⁄2 times makes for a Hard combat.
For example, four heroes (Defenses 6x4=24) enter a crypt. Sarcophagi are on the floor and bones are piled in corners. Suddenly the corpses rise up and attack! There are 2 mummies and 1 skeleton (Defenses 9+9 +6=24), making this fight Average difficulty. Adding 2 more skeletons would make it Hard.
Deadly enemies are different. The usual rule about adding Defense values does not apply. They are so tough, you should only use one Deadly enemy at a time.