Flash, Chris
Chris, liquid volume can be measured just like air with an orifice in a pipe and even pito tube.
Flash
If you use a simple GM distributor and take the condenser out so it is just a switch, you now have a switching mechanism. RPM will differ the frequency (how often the injector pulses), you can then use the point adjustment for Dwell (Points closed) to adjust fluid flow time. As Tom said this is not a true ratio but you should be able to find a sweet spot. To get your flow rate you use a graduated cylinder say 250ML and a stop watch, you then set your motor rpm and dwell, then with the power to the injector off and a clean graduated cylinder you turn on the power for say 30 seconds then off. You will have filled the cylinder a specific amount, then repeat this same test for multiple dwell and rpm setting building a chart that will then be converted to liquid mass for your pseudo air fuel ratio. Remember the liquid pressure at the injector is to be a constant or all this is for not.
You will then need to do similar testing on your flow bench where you know exactly how much air is moving through the bench at any given time (Easy if you are using the PTS-DM).
SO to perform your test as you want you would pick a given rpm air flow for testing say 5000rpm at 350cid CFM = (350 * (5000/2))/1728) or 503CFM now since you are only doing 1 port 503/8 = 63cfm +-. To reach the stoichiometric ratio you would then need the following.
1 cubic foot = 28,316.8467 cubic centimeters.
Dry air weighs 34,093.48 mg per cu.ft.
Or 34.1 grams Which is about 1.2 ounces per cu.ft.
If we keep this simple and just said you were using water as your fluid then we could take 63 cfm * 34grams = 2112 grams/min of air mass divided by the ratio 14.7:1 or 2112/14.7= 143.67 grams/min of water since on gram of water = 1 milliliter = 1CC for us Yanks that would be (1oz. = 29.57ml) so 143.67/29.57 = 4.86 Oz./min at 63cfm
Now if you have any electronic skills you could build a simple tool as Bruce pointed out in his link and it would be a lot simpler, also some manufactures made these tools I have one made By Carter Fuel system I got back when I worked for Federal Mogul. I will post a picture tonight to you can each for one.
Not sure what this all gets you but it is the basic concept you were talking about.
Rick