There are quick and easy (and sufficiently accurate) approximations like the "clock-face": wind direction up to 15 degrees off runway heading: crosswind component is 1/4 of wind (15 minutes = 1/4 hour); for 20 degrees off it's 1/3, for 30 degrees off it's 1/2, for 45 degrees it's 3/4 and for 60 or more degrees one can calculate the full windspeed as crosswind. The pros might have even better approximations, so I don't think anyone has to look at an FMC for that
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