Wayne Walker's recipe for Root Beer Float, from YT:
Root Beer (Flv) 5%
Super Sweet (Cap) 0.25%
Vanilla Bean (Flv) 0.5%
Vanilla Bean Ice Cream (FW) 1.75%
I'd be amazed if 5% of Flv Root Beer is actually needed, but that's his recipe. Flv says mix at 1%-5% in single flavor, but then there's a review on their website that says 2% is plenty as a single flavor. Going by that, I'd guess with creams, maybe 3-3.5% tops. I'm not going to buy Flv Root Beer to find out, though.
Might try the guts of this, adapted to flavors I have, for example maybe 5% of FW Root Beer and 1.5% of TFA Vanilla Swirl. I have the other two flavors.