In various (dis)order :
I may had a vocabulary-related brain fart.
To clarify what I had in mind, I am building a database of wich the main access will be an internet page, but I will eventually later add a phone-specific access.
I only have a dumbphone but I'm pretty sure you can access internet on a smartphone, no ?
So the main purpose is really to note down your training progress, with a option to use the data for scientific use, in an anonymoius manner obviously (well, if you are a slinger with a 30cm armspan, using a 2,50 meters sling to shoot a cube of plutonium of 12 cm... it will be quite easy to follow your progress even without your name :p )

The main point is the target. I thing I will go with Teg's proposition, but with a twist : I will add the possibility to note your long distance training or speed measurement. I don't think anyone is doing one of those while target slinging ?
JudoP and Sarosh, will it respond to your demand ? because I must admit I am pretty sure to NOT understand what you are speaking about
I also thought of something in the middle of the night : in archery, you can shoot at 3D animal forms, does it sound a good idea to you to add this possibility ? I can pre-select a selection of animal to allow you to choose from them.

Frondeur : good idea, I will add it as an option ^^

Lusitano : I planned to do something like a "friend" option where you allow friends to see your records, or you could organise mini-competitions

Jaegoor : It could be useful, as a comparison. Could you please attach it to a post ? or send me a PM ? Thanks !

woodssj : I think I will now use "lost to the winds", there is a poetic air to it, something nautical that feel nice ^^ even if you loose the negative side of the cabbage pile

NooneofConsequence : You change your sling in middle of session to "reset" your feel for the sling you are using, right ? then you resume shooting with the first one ? If it's the case, do you count the hits with the reseting sling or only the "main" sling ?

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Someone once said to me that a good conceived database is half the work done... but we are working to create a monster :p

DOA=arctan(diameter/range)  ( solid angle)
by performing DOA measurements you know if there is any improvement in consistency.
whereas if you measure 10% hits on a  balearic diana you don't know if you are consistent or not, you might be 1 or 90 DOA.

100% hits on a balearic diana (0.5mØ) at 20m would be " 1.43 DOA slinger "
100% hits on a 10cm Ø circle at 20m would be "0.28DOA slinger" and because it's a small number we can start talking about MOA 0.28 DOA = 16.8MOA

It's a better way to measure consistency because it's independent of distance and it's a little nicer and more understandable than saying 0.025 diameter/range ratio or saying a score . It's very easy to compare with other slingers , it's not depended on targets/ ranges.

It's similar to how rifle/airgun accuracy is measured , stats from groupings. Another similar measurement is CEP  https://en.wikipedia.org/wiki/Circular_error_probable 
and there are many other statistic measurements you can make or even graphs but DOA is just one number that can communicate just what you need in slinging. It's a measurement to rule them all.

There is one and big drawback though: you need to make a grouping image and that is not easy for slings.