Bob and Rick always use simple methods to ensure fewer mistakes. Their rule of thumb about dado's is to make them half the thickness of the material. If you use the same method every time you will never go wrong. As far as your fit problem, it is very possible that your 3/4" bit is not on size. There is a tolerance range for bits being acceptable, yours could be on the low end or slightly undersized. As long as you are aware of this you can adjust your routing guide and make a second pass for a perfect fit. You would mark your exact cut size, make a pass on one side and then reverse your set up to cut the other side to the mark.
"Living in the D" (this means Detroit!)
"It's fine to disagree with other members as long as you respect their opinions"