You may want to think about getting a new slide and do a leak down test. 52 pilot is MASSIVE. Stock pilot is 22.5. Without knowing much else -if you're running an airbox lid, porting, have an airleak, etc - i'd say 380 is fairly close. My guess would be 400 - 380 if your bike has an '88-'90 jug on it. With a pilot of 27.5-30. But than again, your slide looks crazy worn and that will affect the pilot size.
Only buy genuine Mikuni jets. Also, buy a few sizes and start big and work your way down.
You don't have to be an expert, but you will learn. Trust me, very few shops know how to jet well or even work on a 2stroke. Let alone a zilla.
First things first. If it were me, i'd do a leak down test first. If you have air leaks in the motor, you will never be able to tune it.
If you dont know what a leak down test is, here is an example of one.
http://www.suzukiquadracerhq.com/lt250/lt250r-leak-down-test/msg47754/#msg47754You can literally build one at lowes for less than 20 bucks although you may have a hard time finding a gauge that one reads about 10 - 20 psi. You only need to pump in about 5-7 psi.