I'd try 70k alt first, but I suspect you can get a little lower. Haven't had a chance to check. If you come in inverted (and at that speed you'll skip out if you don't) start with a zero AoA, and use elevator trim to hold at 70k.
If you want to try a higher AoA, start with about 55 or 60 degrees inverted, otherwise you'll have too much lift and get pulled down into the atmosphere and burn up. There's a trick to getting both RCS and aerodynamic control surfaces working simultaneously - you'll need to do that to maintain the high AoA. First, while in Panel View, set the controls to AF. Then switch to "glass cockpit" mode using <F8>, and click on the RCS ROT button on the top left of the screen. As you get down to 70k, lower the AoA to increase lift so you can stay at 70k. Watch your VS and VACC closely. Expect to burn up a few times until you get this right.
I recommended the zero AoA and low AoA during the initial braking phase because it's easier to do, and easier to transition to upright when you get down to suborbital speeds. It does take longer - you get much less braking that way.
If you do go with the higher AoA, drop as low as you can until you get to about 8.25 Km/s, then start climbing back up to 70k. You'll want about 100 m/s of positive vertical speed when you transition to the low AoA before turning upright since you'll be passing through the DGIV's maximum lift AoA and it will pull you down.
You may want to check out my Home Direct tutorial, it shows the inverted aerobraking in an XR2. It'll be a little harder in a DGIV since you cant shift the COG, but the principle is the same.
[ame="http://www.orbithangar.com/searchid.php?ID=3660"]Home Direct[/ame]
