It's a matter of choice and everyone is different.
But the rule of thumb is that the lower the number, the hotter the run and the more vapor and flavor they produce. But as the number gets lower, you run the risk of "burning" the
juice which tastes terrible. Higher numbers are used for PV's which work at higher voltages like 5 volts.
For a Riva, I would suggest that you use 2.8-3.0. I like a smooth hit and 2.8-3.0 is a good trade off between vapor/flavor and burnt taste.
You will also find that it will depend a lot on the
juice you use. My DIY Pineapple and Watermelon taste the best at 3.0-3.2 where as my DIY Irish Cream taste better at 2.8-3.0.
Your going to have to do some experiementing.