[Bruno]

While nothing substitutes for actually running the battle, and especially running the battle with the actual players involved, sometimes you need something to set themselves up in the right ballpark in the first place (and some people need it all the time).

The downside, demonstrated by D&D, is that any kind of quick and useful guideline tends to be taken as sacred gospel and a hard rule in practice by some people - but GURPS has never taken the lowest common denominator as their target in the past, and I don't see why we should fear it.

Yes, terrain, specific party composition attitude and play style, tactics used today etc radically changes the outcome of the fight. That's why you actually PLAY it - if everything is foregone, there's no point in rolling dice.

The same fight can be a cakewalk if the players organize themselves and use good tactics, and a massacre if they play like idiots. There's not a lot a GM can do about that, other than guessing where on the briliant-stupid axis the players will land, and possibly adjusting the monsters/encounter mid-game if his guestimate was off (depending on the GMs play style - some will fudge dice rolls, some will rewrite stats, some will have monsters flee or arrive for reinforcements, and some will play it as written and the PCs will just have to live with it or die with it as it may be).

All the dry runs in the world won't help if the GM has missed a tactic, or if the players are playing goofy, and angsting over that in advance tends to not be worth the time and stress, in my experience. The 10-20 minutes to eyeball the numbers has a high time-to-result payoff, plus perhaps another 5-10 minutes just casually thinking the fight through.