Using total levels is a start, but I have found it by itself is not enough, because a solo creatures tend not to be as challenging as groups of creatures. If the party is 5 level 1 PCs, then a single level 5 creature will generally (IMHO) not challenge them as much as 5 level 1 orcs, simply because a single high roll on an attack or critical can end the level 5 creature's career, whereas the five level 1 creatures each get a chance to land one open-ended attack or high critical. In short, the swinginess of RM combat rolls means that it is almost always better (statistically speaking) to have five +30 attacks than a single +80 attack. So the number of combatants must also be taken into account. Also, if there is only one monster and 5 PCs, the PCs can work to make sure one or two of them are in the monster's face and parrying while the rest attack, greatly reducing the chance of a solo monster winning. So if I am using a solo monster, I can make it a level equal to the total level of the party.
If I am using multiple creatures, I rarely put a number of them equal to the total party's level. A group of 5 level 1 orcs is going to be a real challenge for a group of 5 level 1 PCs, especially if those orcs have OBs similar to those of the PCs, so most battles will have a bit less than that-- say 3 or 4 level 1 orcs.