Is there a fast and loose way to scale an encounter to a changing group size?

If I have a starting group of 6 and 2 are unable to make the session, how would I scale the challenge up or down?

Short of hurling massed Minion waves of setting up Rivals as, in effect minions, seems a bit cheep.

Any suggestions?

Just take a look at how many target groups your group of players can attack in a single round. Anytime you exceed that number you will be challenging them as there is someone they can't shoot/hit. That number is going to vary of course based on what kinds of attack methods they have. Then just use whatever meets their relative level of damage output. Some of this requires having an idea of the kind of damage they typically deal out. You also have to have an idea of what level of damage they can soak up. You can mock roll it for yourself to get a feel for how they will fare prior to get a feel for how it will play out.

I'd start out by scaling it to, say, four players (depending on how often there are players missing from your table). Then merely make a note that for each additional player beyond four, you add another Rival or two Minions (or whatever is appropriate for the encounter difficulty) to the mix. Use the same stats that you already have for the NPCs in the encounter, and you shouldn't have any extra work at all.

From my experience Usually a Rival per PC is a good balance. A group of 3 to 4 minions per PC (they pretty much become useless after they get hit a few times). Really depends on the Nemisis, but less then 1 per PC.

Run a couple of encounters and start scaling up until you have know you have a good challenge for your players. That is part of being a good GM

My 2 cents: Underestimate until you get the hang of it. It's better to have the players mow the enemy down like an unkept lawn then have them get owned by some mooks.

My 2 cents: Underestimate until you get the hang of it. It's better to have the players mow the enemy down like an unkept lawn then have them get owned by some mooks.

Absolutely, you have to have that idea of what they can take and dish out before you can tailor encounters really well.

For example, i have 3 players (Merc soldier, Thief, Politico) they usually have no problem with minions, after the first one or two go down they are pretty much fodder. WHen I introduce a high soak NPC they start having troubles since it would take them several hits before they would go down

It is okay for your players to mow them down. Players LOVE THAT. But also there is a special thrill in getting through a battle and everyone is wounded, has a few criticals, but still manager to win

It doesn't matter. It takes a degree of effort to kill a player in this game, unless the player is very unlucky they can take several critical hits to kill them. Just exceeding their wound threashold does nothing more than a critical.

So you hit the players with an encounter that wipes them, probably what you have done is exceeded their threashold and put them down with 1-2 criticals, not much more? Well, have the players wake up, tied to a pole in the middle of an areana with huge beasts heading towards them. As long as the means of escape is there and exciting your players won't or rather shouldn't mind having this happen now and again.

(This was good enough for George, so as GM's we should get away with it now and again.)

I think as long as the players have the choice to fight or flight, even if you hit them too hard the campaign will continue and probably be far more exciting for the players as they will never be too sure that they will win each and every fight.