Setting up slitting saw for round stock?

How do you propose to set it on "center"? (English speakers spell that as, "centre"!). That is the nub of my enquiry.

Well, if you have "stock", and it's a small thing of which you're making many, you get a method that allows both measured increments and a locking system. You do a test, and adjust, so that once it's good enough, you just shove the stock in and slice it.

If it's a one-off that has much time/money/work already into it, you use the same arrangement but plan and measure as best you can first.

Stupid boy.

I've got some that are about 0.3 inches in diameter.

You might get a better result with less trouble by using tapered plugs rather than cross-cut pins. Get a #4 taper pin reamer to ream the "jacks", set up your lathe to cut the mating taper on the plugs. The nominal dia of the large end of a #4 taper pin is .250" The taper is .250" per foot. This is about 1.19 deg included angle, which would be a "self locking" taper providing very good contact if you got the taper right.

You could also just buy taper pins, solder them to the ends of bits of shaft and copper-plate them.

There are various methods. First, you have to specify what kind of equipment you are using. You never did say. BTW, American speakers spell it center. We left England so we were free to do it a better way.