How much do missiles weaken the SDI? Is it a set amount or a portion?

its a %.

how much of a percentage?

1%

That is it takes out a percentage of the tech points, it's not like you DH someone at 90% SDI and he is then 89% SDI.

Because tech effect is not linear the effect on the percentage is much lower than that.

But you can still calculate that around 70 defence held missiles will lower the targets SDI tech points to about half its starting point.

Slagpit sighting! *waves*