Still depends on how fast you swing, how fast the incoming balls can get, what you like in feel, your swingpaths, and probably a handful of other things.
I have 7 rackets with STBite 16 strung at 35 lbs. That's the lowest the stringer will do. Sometimes it feels a little stiff, so I stand on the stringbed to lower the tension around 3 lbs., which lasts about 4 minutes before it starts feeling a bit tight again.
I"m old, weak, don't swing full speed for groundies, but still serve around 100 on flat first serves.
I know you don't believe the last sentence. I've played well OVER 20 guys on this forum, and not one, including a few 5.0's, would ever say otherwise. Lastest to face some of my moderate speed into the sun lefty flat serves is Erlang on travel section on this site.