My Ultegra brakes have a cupped washer system. Now, it's not really a true cupped washer. The brake pad cartridge, the metal piece that holds the rubber pad, has a raised circular section where the bolt attaches. That should give you some adjustment. The picture should make that clear. That rounded curved part should give you more than enough range of movement to adjust the toe in.
If the brakes are moving out of position while you tighten the screw, put something hard (but something that wont scratch or dent your rims) and thick enough against the brakes as a shim to give you the needed toe in and apply the brakes against the rim. Make sure everything is aligned and toed properly Tighten the pad holder bolts. The pressure against the rim should prevent the brake pad holders from moving while you tighten.