It's approximate.
You might want to take 1013 as sea level, and work 27ft per millibar up from there. But that works only at low levels, and the 27ft/mb soon becomes 50ft or even more. The vertical scale on this skew-t gives you the general idea
I don't know if there is an online converter for millibars to altitude, and in any case it would vary on the day according to the "thickness" property. Typically, 500mb is about 5300m (17000ft) and if you look at a typical MSLP chart you can see the thickness parameter shows in decameters e.g. here
you see the "thickness" as a dashed line at 564 decameters and this is the 500mb surface. This is all affected by temperature also. But for wind estimates you don't need to be that accurate.