Blinding road users will be the result of the following factors:
- Total light output
- Mirror design (how is the light shaped)
- How you aimed your light
Most trail lights (and high output battery powered lights) use a mirror that casts the light in a symmetrical shape. This means light is cast up, down, left and right. Light cast above the horizon is what binds other road users. Light above the horizon is however great for trail riding as it lets you see overhanging branches and other impediments.
Car running lights (i.e. not high beams) are designed with a horizontal cut-off, where light does not shine above the horizon (actually a low percentage does, this called spill light). If the car light is aimed correctly, the the horizontal cut-off falls below oncoming road users line of vision. This is why you are typically not horribly blinded by oncoming cars, if they are a) using their running lights and b) have their lights aimed correctly. High beams do not have a horizontal cut-off, which is the primary reason you get blinded so badly.
Some bike lights (those that are approved for German roads) will have a mirror that has this sharp horizontal cut-off. They usually will usually be indicated by the some statement on StVZO compliance. Most dynamo lights (e.g., B & M, Schmidt, etc) will have this mirror design. There are also a few battery powered lights (e.g., some B & M models), but they are few and far between.
If you have a light with a symmetrical beam, you can lessen the blinding effect by pointing the light down (as suggested in @Batman's answer), however due to the symmetrical nature of the beam you will still blind road users more than if you had a light with a horizontal cutoff and was aimed correctly as there will still be more spill light above the horizon. This effect is also confounded by how narrowly or wide the symmetrical beam is focused. Wide flood lights will almost always blind, despite how much you aim them down, spot lights will blind less when pointed down.
Finally, total output does have some impact too, but it is also confounded with mirror design (e.g., high output with a horizontal cut-off may blind less than a low output symmetrical beam). What really determines whether or not you blind other users is the total amount of light shining above the horizon at their eyes.
In your case your 750 lumen lights are likely trail lights with a wide symmetrical beam. If you run them at full output you will likely blind others unless you severely aim it downwards.