I have spent about 4 months on 2m antenna test range playing with Yagi -
Quad - Razor (Quagi) designs. (before modeling software was available)
Design objectives were to obtain best gain, F/B - clean pattern aand 50 ohm
impedance.

My findings we

The maximum performance for 3 el. antenna was obtained from 3 el Quad.
The rule seems to be quad elements work best with one on the front and one
behind the driven quad element or log cell.

For 4 el. parasitic antenna it was 3 Q and 1 Y element.

For 5 and 6 el it was 3 Q 2 (3) Y

For 7 el the winning combination was Y refl. Q refl. 2Q log driven cell, Q
dir. 2x Y dir.

For over 7 el. I would just keep adding Yagi directors.

See http://www.k3bu.us/razor_beams.htm for some pictures on the cover CQ
Magazine featuring stacked 15 m 7 el Razors.

Hope it helps.

73 Yuri, K3BU, VE3BMV