My successes in level bombing are kinda varying, better results when I am 'concentrated', and keep my air speed and altitude constant.
I never use that method he uses to set the TAS, I use this bombsight calculator: bombsightcal0.33
(Well, due to not paying attention, sometimes I franticly try and fix the bomb sight on the target using inc/dec of TAS setting, but that never leads to a good hit, maybe once in a full moon if I am lucky!)
I run the calculator on a separate Pc, but it should be possible to run it on the box you run IL2 on... not too sure! Usually I do the calculations before the run, preferably during briefing, as alt and speed should be known at that time. Circumstances during the mission beyond my control may lead to a re-calc being neccesary ...
If you drop long all of the time... maybe setting the TAS is incorrect?
Do you take the target altitude into the calculations? Though if you do not do that, it would mean a short drop when the target is higher than sea level...
Also, wind might be a factor? Head wind/tail wind? This video does not mention side slip setting, which you will need should there be some wind in the mission. BombCalc does cover this...
And yes, keep practising, using level stab does not always guarantee a fixed height and speed!