Video Doorbell Pro 2 - Transformer Question

Just installed Video Doorbell Pro 2. Device Health shows Transformer Voltage Poor (15V) and power output level 2. If i try to turn on package alerts it says feature requires more power. Transformer is original to house (1962). I did a voltage check before installation and i was getting 17-18 volts. I would imagine my old transformer doesnt have enough amperage out. Transformer is in the attic ( i think) so i want to replace once with the correct one. Any recommendations? I know the Pro 2 Wired takes more power ( 16-24 VAC 50/60HZ, **30-40VA max). The one ring sells looks to be 16v-30VA. Would i be better getting a 16v-40VA or maybe a 24v - 40VA?