run it with the whatever needles are in them first (ID is stamped on the needle's base) to get some sort of idea where you stand. Then depending on wether its lean up top or whatever you can choose different needles.

Any decent carb parts shop (or MG/Rover specialist) will have a listing of needles and their calibrations which will be able to help identify a suitable needle. SU needles are metered at certain increments for the first 1.5" from the shoulder, any length past that is just waste material. So for example, if it is lean up top, you just need to look for a needle with similar calibration for the first 1/3 and gradually thinner (richer) for the rest.

How far you had to wind the jet down (to make it richer) to correct it's mixtures at high rev/load (that would obviously make it too rich at low speed, but this is just for sorting purposes) will give you an idea how much richer the new needles need to be, because each full turn of the jet moves it down a fixed amount equal to the pitch of the thread (which I can't remember now - 1.5mm?) and that will tell you where you're at on the current needle's chart.

don't know if that made sense, it's a bit hard to explain just in words, but its alot easier than it probably sounds