Diggidy
There are many low cost tach's out there, I believe I have seen them for about $35.00.
Yes I am still using 2 cells, I find the 2 cell fits everywhere, AB, Formosa, Corsair, etc.
What motor are you using on the AB, I forget, stock? Motor Max? or the Mega?
At 12.6 volts ( full charge), the stock motor would cook in a short time, the Motor Max would take longer to cook but it would overheat, as for the Mega, I don't know.
With my Himax motor running a 14X8 prop and a 2 cell lipo ( motor draws 15 amps) in the Formosa I get INSANE flights, I have max throws on the ailerons and get unlimited spinning while climbing vertically, the only limitation is my skill level.
The Formosa flys good at low speed, when new and not loaded with epoxy it floats rather well.
My first videos with the Formosa ( around September) showed this, I would suggest you do the first few flights using the stock motor with reduced throws, it will fly like the AB, just a tad more responsive.
The only motor we used that made the AB go vertical was the Himax.
Next time I go out with Mary Kate, I'm going to see what my best climb rate is, she comes in at about 15 oz, the Motor Max 400 T with the 9X4.7 gives me 14 oz thrust.
If you watch my current video
http://users.adelphia.net/~afineman/ you will see with the first toss she wanted to go straight up, I'm starting to come around to accept what some of the old timers preach, Weight NOT power.
But until I get some more flights in with Mary Kate I won't recommend anyone else do what I did to it.
Hope it helps