To decide how much it's lowered, most of us just eyeball the height of the front wheel arch versus the wheel/tire and decide what looks good. The other factor is whether the tire rubs against the underside of the body on hard dips.
As far as measuring somewhere, all cars and wheel/tire arrangements are different so what I show (with my 205/16 tires/wheels) will be different from what you see.
What, exactly, is interfering with the intended anti-sway bar, that big clevis-like thing on the bottom tube?
"Lowered" shocks are simply shorter versions of the stock shocks (sometimes slightly larger diameter, too). One problem might be that they don't allow as much downward extension of the suspension (because they're shorter). This may be an issue or not. You won't hit the suspension stops if the car body lifts - You'll reach max extension of the shock, first. I'm running my suspension pretty low on stock spindles (not "drop" spindles) with stock length shocks and everything seems to've been happy for the past 20 years.