Optimal injection volumes are directly related to the cylinder volume of your column and are, therefore, dependent on the cross-sectional area (A=π r2) and length (L) of your column. Since that is the case, you can estimate any adjustment from an existing method for injection volume.
If you are converting to a different size ID (with packing material and length remaining the same), just multiply your current volume by the ratio of the radii squared to determine the correct volume for your new method. For example, if you are currently injecting 20 µL on a 150 x 4.6 mm column and then switch to a 150 x 3.0 mm column, you could estimate the adjusted volume by multiplying 20 x (1.52)/(2.32). Your new volume should be about 8.5 µL.
