I would turn the flash off, meter the scene with your body, then switch into manual mode with those settings, and set your TTL flash to -1.7EV, then I would bump up the falsh in .3 steps to a max of -.7EV and see which gives the proper effect.
When you over-expose are you using TTL-BL or TTL with -EV comp?
Also, are you pointing your flash head right at their face? Try pointing it up or at 45 degrees instead and pull out the bounce card.
Could you tell us what flash mode you are in, etc?
Perhaps a ND filter might help?
Shooting wide open in daylight will give you some very high sutter speeds. A side effect is that you'll be greatly exceeding the flash sync speed. A ND filter will cut down the light the camera sees (kinda like sunglasses) so you can shoot with a larger aperture and add flash and keep the shutter speed lower, all without overexposure.
I second twig's suggestion of bouncing some of the flash. If the light is too harsh and blowing out the face, it sounds like that could work.
I assume your interesed in fill flash otherwise I use the straight iTTl settings. I have a D2h and SB800 and for fill I comp -1 to -1 1/2 on the flash and see how it works. It depends if it's in direct sunlight or shade. You also have the BL option to play with.
I have a D100 and SB800. For fill flash, I use the SB800 in D-TTL mode. For normal photography, with 3D matrix metering, depending on the lighting conditions (direct sunlight or semi-shade) and the distance to subject, I compensate by -0.7EV to -1.3EV. For macro and closeups, with center-weighted metering, I use a Wizard pocket diffuser over the flash head and compensate by -0.3EV to -1.0 EV.