Even if you don't break out spreadsheets or game-specific calculators or whatever, you develop an intuition over time for what looks right as far as stage sizes, payload-to-fuel ratios, that kind of thing. I've done it all by eye & guesswork and have had successful out-and-back missions as far as Minmus and solar orbit (but my not taking it seriously enough and running some numbers ahead of time is probably why my attempts to reach the Mars-alike planet have just been big wastes of time—I just don't have the patience to figure out the ideal launch window and run the time ahead to then, to work out a proper budget for my fuel, and all that—plus those missions are easier with orbital rendezvous and I hate those, I can do them but they're such a pain in the ass)