You use Germany or what other civ?Getting a city to 100 production is not that hard and I have that in almost every game. I also had a city with 150+ production in some games, but that needs lots of trade routes and some luck with hills or forest hills near rivers.
(I remember having 200+ production, but I guess that was before the factory nerf.)
I would say ~100 is about average for my capital in a science victory. For other victory types its a ton lower, usually around 50. No real tricks, just move trade routes, keep up with housing and amenities, get industrial zone, encampment, don't build settlers from capital. Globalization obviously gives you a huge boost but I'm usually winning science games before then these days.