5 Parry = 1 deflection AC, modified down ala tumble (ie, you take lower, RAW or modified). This would give +1 Deflection AC at lvl 1, +2 at 7, and +3 at 12 (lol)
Deflection AC does not stack. Deflection AC is easily acquired. It would be a small change, and make Parry less of an absolutely useless choice except for that one time in EfU:M