Around 1999-2000 we were doing these kinds of calculations, and 13% seems a little low.
Back then, I'm pretty sure that around 20% to 25% of all the btu's going into the cylinder were from alcohol. This was based on 6 blue top injectors at 100% DC and 45 psi FP, stock motor, and we flow-tested our alky nozzles into a graduated cylinder. At the time we were running anywhere between 25and 28-29 psi boost on a stock block/head/cam motor.
I'm not sure what the number works out to today - I've since changed injectors, turbo, heads, intercooler, added more alky nozzles.
One simple way to ensure you are not too lean would be to gather a baseline of max EGTs on gasoline. Then turn on the alky, and watch the EGTs. In my world, 1500 deg F is lazy, 1600 deg F is good and reliable, and if you can control the knock, 1700 deg F egt is hell on wheels. Measured at inlet to turbine.
Hope this helps.
ps - Nice old-school Larry Fisher knock detector reference! He used to live not far from here. Had a sweet 87 GN with GNX dash, and his electronics lab in the basement. Now he drives a C4 or C5 I think..