Quick thoughts.
THIS IS AWESOME. Know that the tool you actually develop is less important than the fact that you (a leader) and your team are engaging in the process of intentional design for improvement. Keep it up :)
Where you're on the right track: trying to create a measurable that can be tracked, and breaking it down into smaller chunks, variety of throws/situations.
I'd say continue to know/evaluate your limitations and exactly what you're measuring, go for granularity, which is much more useful to players (and anyone trying to learn a complex skill). You don't want them talking about the 59 they scored, you want them talking about how they were 0/5 on their backhand huck, or 5/5 on their io forehand. This allows them to know where to improve their game, or what throws they can be confident with in games.
Tough to measure... breaks add in tons of uncontrollables and variables, careful how you measure. That's not to say don't measure it, just know that measuring it is... hard. Maybe focus more on throw shapes rather than the mark (i.e. IO forehand/backhand)?
On the number system... All the evidence about learning says that a key step to improvement is making results measurable. That said, numbers aren't always best tool. Rubrics are often the most effective tool for teaching and learning, because it can give you not only where you're at, but describe what you need to do to get to next level.
Try to integrate self evaluation into your process. 1/5 on your upline forehand pass isn't enough data to help you know exactly how to improve. Realizing that 4/5 throws zipped past the reciever or 4/5 throws were behind the reciever is what will help you make improvements.
As for language on your rubrics... I've started using the words beginner, intermediate, advanced, elite as the foundation of all my rubrics, because these words are commonly used throughout the world. You can start with Elite then work backwards taking away levels of execution.
Upline backhand throws
Elite - Consistently leads receiver in stride with an easily catchable throw. Consistently completes passes to multiple different cut angles and distances. Executes consistently under pressure (i.e. high pressure standard mark any weather condition, etc.).
Advanced - Leads receiver in stride well with catchable throw most of the time. Completes passes to multiple angles and distances most of the time. Executes consistently in most pressure situations, but not all.
Intermediate - Moderate but inconsistent execution, sometimes a very difficult catch, sometimes not in stride. Inconsistent when throwing to multiple angles and distances. Pressure situations affect execution.
Beginner - Inconsistent execution, not in stride, not catchable. Only can complete at specific ideal angles and distances. Pressure situations make execution extremely low.
Again, awesome you're building something... and remember, the building/construction process is likely the most valuable takeaway, the tool will be refined over time when you've used it and know it's benefits/limitations!