The only nitrous experience I have is my street driven '70 Camaro. Nothing on methanol or mechanically injected.
I do know how it works and your idea for enrichment sounds just fine to me. You're doing the same thing as a nitro car with a "High-side/Low-side" setup. You'll run with an extra bypass to take away fuel and run on the "Low-side" until the nitrous kicks in...then you take away a bypass orifice by closing a valve and you're on the "high-side" sending more fuel to the motor.
Just like a nitro car, you'll need to discover what bypass area to use for both circuits.
For a cheap solution, you could look on eBay or scientific surplus sites for "rotameter" type flow meters. It's a graduated tapered glass tube in a housing with a steel float inside. The float indicates flow. You can frequently find them used for under a $100.
http://en.wikipedia.org/wiki/Rotameter A more expensive (but still affordable) solution is to look on Omega or Cole-Parmer's website. A single sensor will run $300-600 and you'll need to send the pulses to some kind of panel meter that you can program to read GPM. Basically, a programmable frequency meter...you'll tell it how many pulses per gallon and all that.
Instead of getting all wrapped up in building a flow bench, you could just send the work to a shop like mine. Send the nozzles and pump and request that you want X.XX amount fuel to go through the nozzles with ONE pill (high-side). Then request that another bypass pill be added until it flows Y.YY GPM. The flow numbers can be captured and sent along with the pills to make it happen. The whole thing should cost less than $300 easy.
Spud
