I am pretty sure that you have to have at least 200+ uses in one day. I did this with 2 boost accounts, and used the BMW 1 Series M Coupe you get this car for free if you pick it at the start of the Career. I did about 150 on one of my dummy accounts, and then 50-60 give or take with my other dummy account. I did all of this after I received the messages from Forza about more uses can earn you money. I stayed up until 7am I got about 200+ uses for my tune and design but I didn't want all that work to go to waste so I asked a few friends to use my design and tune when they had the time,1 guy used them 21 time the other 20+.I did this because I receive my messages from turn 10 at 8pm and I started boosting my uses a little before 12am and I did not know if that would have an effect on my achievements because I started before 12am I didn't know if X amount of kills before or after would be added for my day grand total, so I put the original 200 up to 250+.All this put me at a grand total of 255+ tune and design uses, and 4-5 downloads total. The next day when it was message time I did not receive anything but the same old messages about more uses will gain you more money.I was very mad and didn't want to boost it anymore so that day I didn't do anything with my tunes or designs and no one else used them. The next day came (12/02/2013) a friend told me to check my turn 10 messages and I received the following:
Professional status: In the last 24 hours, you earned 51,289 CR for your shared tuning.
Professional status: In the last 24 hours, you earned 35,545 CR for your shared
Now I did have a Audi that had 200+ uses for tune and designs also but that was done a long time before this.
So I'll I can say is I think you need 200-250 uses in one day maybe 100 uses but I tried 250 and it worked for me.
So If you want to do this by yourself here is what you do:
Make 1 or 2 extra accounts they become gold once you get them on your Xbox one, if you already have a gold account on you system.
buy a car on you main account that you can get for free on you dummy accounts at the start of career.
Keep it simple use the BMW 1 series M coupe.After you buy this car make a design, and tune just change the tyre pressure keep it all free :) because your dummy accounts will not have any money. Make sure to give your design and tune a simple description to make it easy to find.
Now start the game with 1 of you dummy accounts and do the race in the P1 after this is done choose the BMW 1 series M coupe as your free car or whatever car you happened to pick. Now after you get your free car you can pick you paint job hit the back button to search for you design, once you have it continue. The game will put you in a new race, once it starts pause the game and quit out to the main menu.
Go to tune/upgrade and hit X now hit back for setup manager, select load setup, hit back again for search, find you tune and save and install it. Back out to the main menu and select Free play, select your car with your design and tune on it. set the game up like this:
Tack: Top Gear, West Circuit
AI: 7 or whatever you like
Collision Mode: Always off
Now start game do your one lap, make sure you select restart after you finish each race this will save you lots of time trust me lol.
Now I hope this helps you guys and gals out, If anyone wants to try to do only 100 uses in one day and let me know what happens that would be great.