What are the optimal percentages for analog system

This is a little complicated especially not knowing your budget and how far in your upgrading you are prepared to go in the near future. But I agree with those who suggest you get the best table you can. Good $200 cartridge will sound OK with $2000 table, $1000 arm and $400/$500 phono stage. With decent cables of course. You could also consider voltage stabilizer/conditioner or/and motor controller. Turntable can last for a very long time.
I personally would probably wait and upgrade everything at once and would spend at least 50% on the table alone.

Where should most of the money go...cartridge, phonostage, or table?

Imo, the priority goes like this: Table, Arm then Phonostage then Cartridge. For now, with your current gear, I agree with Elizabeth. What price range were you thinking?

There are no hard and fast rules but something like this might be a fair starting point:

Table/Arm- 50%
Phono Preamp- 35%
Cartridge- 15%

IMO
YMMV

Thanks for your responses. It seems like I should start with upgrading the phonostage first. I'm looking to spend about $500 on a used unit.

If you're a Grado fan, with your current system I would first audition the Grado PH-1 phono stage. I've never heard anything better with their own cartridges. Next, I'd look at upgrading the turntable. The Well Tempered Record Player is a great, modestly-priced platform for Grado cartridges, and the Grado tracks like a champ in the Record Player arm. In terms of PRaT, the WTRP offers a somewhat different balance of strengths, with pacing its particular virtue but with still very good rhythm; like the Rega, timing is a relative weakness. After that, I could recommend the budget-end Statement Platinum1--the low-output Grado really does offer some refinements worth hearing.