To get a rough indication, use the Audi finance calculator tool here and do the figures on the Audi S5 Coupe as this will have very similar comparable numbers.
https://www.audi.co.uk/explore-models/finance-calculator.html#/calculator?range=A5&model=S5 Coupé&trim=S5 Coupé&modelCode=F53S4A\0\MSNRS3C\GYMAYMA&productType=SOLUTIONS&deposit=375&mileage=10&period=36
The Audi S5 is £47,875 but comes with a £2,500 Audi contribution which brings the car down to £45,375. Pretty much the exact starting figure of an RS3 saloon.
Then play around with the deposit vs the amount of credit. If you know your RS3 saloon is going to cost £50k and you're going to put a £5k deposit into it, then you will be obtaining £45k credit. Adjust the S5 personal deposit so the amount you are borrowing is £45k. In this case, put £375 into the personal deposit box so the value of credit shows £45k in the quote.
The GFV is showing as £26.1k which is where the RS3 saloon will sit give or take a few hundred £.
Monthly payments are £725pm over 3yrs 30k miles and 6.2%
This equates to you speccing your RS3 to £50k, paying a £5k deposit and borrowing £45k with a GFV at £26.1k and 6.2%. The payments will pretty much be bang on. If you're paying less than £5k, add on £26pm for each £1k less and if you are paying more than £5k deposit, subtract £26pm from those figures for each additional £1k.
It gives you a rough indication of where your monthlies will be.