How do I price glass?

I've had some trouble in recent games making glass work out for me...

1 glass = 0.5 oxy + 2 silicon

So don't build into glass unless $glass > 0.$oxy + 2$silicon?

 

At least in the games I've been playing I haven't seen much of that...

I've also had trouble supplying the furnaces with oxygen... Is there a way to have auto supply be the default rather than something I need to remember to toggle?