If you have one of the semi automatic bomb sights like in the He 111, Ju 88, B25 etc you don't need to know your true airspeed. You can determine it by trial and error as the bomb sight will show you if your data is wrong.
Just punch in your altitude, take your indicated speed and add some notches depending on your altitude (I usually start with 30 - 50 km/h faster than my IAS at more than 3000m altitude). Now look ahead through the bomb sight and target something (preferably at an angle less than 65° as it seem the bomb sight will work inaccurately beyond that for some reason). Once the bomb sight has been switched to "auto" the cross-hair should remain on your target. If your target moves faster down than your cross-hair your entered speed was too low. Add more speed and re-aim (be careful not to get to the automated bombs release, go for a new dummy target if necessary).
If your cross hair moves faster downwards than your target your speed setting was too high. Lower it and re aim. Do this until your cross hair stays on the target. Minor changes can even be done while the auto mode is active.
Practise that a little any you will rarely miss your targets, even from high altitudes without any sort of calculation.
If you have a "dumb" bomb sight like in the TB3...well...no idea. I think there have been charts on the internet that showed your TAS in relation to IAS and altitude...though I think I still missed with them by quiet some margin....
