I went through a few sets of needles after I installed an AFR gauge in my Z and got a better idea of what was needed. I didn't get too scientific with it, other than making general observations at idle and different throttle positions like you have then looking for a needle profile that was bigger or smaller where adjustment was needed. I'm running a mildly built L28 with SUs. I started with SM needles, which I had to run super rich at idle to get decent AFR everywhere else. I tried SB needles also, but settled on RH needles as the best range for my engine. This is a great website to compare needle sizes https://www.classicminidiy.com/technical/needles. I never found a good source for needles in the states and ordered mine from https://www.sumidel.com/ out of Australia. The conversion rate was decent a few years ago when I ordered, so the price wasn't as bad as their site appears. Good luck!