300K miles on tranny,,question

[Black300cd]

I've only got 299,000 miles on my 83 300CD transmission. It shifts solid from 1st to 2nd, slips a tad on the shift from 2nd to 3rd under medium/light load, slips on the shift from 3rd to 4th with light load. I let up on the accelorator when it is ready to upshift to the next gear to try to eliminate the slip during those shifts.

I'd like to get another 300,000 miles out of this tranny but I doubt that will happen. I've have the bowden wire adjusted about how I like it and will probably have my indie adjust the modulator when I have the tranny serviced to see if he can firm up the 2-3 and 3-4 shifts.
I put a quart of trans-x about 40 miles ago to clean out any varnish before I have the tranny serviced. Yes, I drained some fluid out first so as to keep the level exactly correct.

1) My first question is, how long should I let the trans-x work before I have the transmission serviced?
2) Any other suggestions on what to do to help with the weak shifts?
3) Any suggestions on how to extend the life of this transmission, I'd like to get my moneys worth out of it before a rebuild
I'm mostly kidding on getting much more out of this tranny and I know at some point I'll have it rebuilt or replaced. I'm not sure if I'll rebuild it the next few thousand miles or wait until it gets worse.
I guess when it is time to rebuild, I'll see if my indy can recommend a trans. shop that he would trust if he won't do the job himself. This car has been taken car of by the same mechanic since new and I don't trust just anyone to tinker with it.
Sorry for the long message here. Just trying to get some input from other owners who know and love these cars.
Thanks, Tom