Or if you have a mathematical bent you can use sine calculation. The Dalton computer (or a clock) is a visual representation of the Sine wave.
Using your 20 kt example: the rule of thumb is 30 deg off is Half the xwind (10 kts ) , more than 60 degrees is all of it (20 kts) . For the rest you can interpolate. I tried to put it in a table but i've discovered tables are hard to do in PPRUNE.
Degrees off ______SIN______Resultant xwind
0__________________0_________0*20=_______0kts
30_______________0.5______0.5*20 =____10kts
45_________________0.707_____0.707*20=____14kts
60_______________0.866____0.866 * 20= __17kts
90_________________1_________1*20=_______20kts