Hey @Bill,
If you have the user id /amplitude id in hand, then first step would be to create a chart like this demo one (assuming Add Item to Cart is the custom event equivalent and item id the entered weight).
Once you have this, you can export the csv for this chart or look at the breakdown table. You can then see the distinct values of that weight property collected over the last 30 days for the user.
Alternatively, if you just want the count of those distinct weight values collected on that user , then you can switch to the Properties metric like this and view the distinct property values per user.
Let me know if I have interpreted your use case correctly. Hope this helps!