I'm not emulating reality with this; I was trying to keep it kinda simple so its easier to implement and use.
Something like 'damaged machine hull' is smelted into 7 ingots (7/8ths of the metal used in it).
A machine capable of handling byproducts (2-3 output slots- like as a pulveriser ect) would make this a bit easier; crush up an old circuit to return some piles of iron and copper dust.
(Gtech is a different ball entirely; given you can use different metals for machine hulls. A salvaging mechanic would be awesome, but it'll take a lot more tinkering with the core mod)
I think its more a case of borrowing an idea from reality, and reworking the mechanics to suit the game.