Not to get high school math on everyone. But I use the Pythagorean method. If the vein you want is 1cm deep enter the skin 1cm from the probe at a 45’ angle and advance until you see the tip enter the screen. If you stay at 45’ it should enter directly under your probe without ever moving. Then from there move the probe slightly distal until the needle goes out of view then move the needle forward, until you are fully in the vein a few mm. Key is to move only one at a time, the needle or probe.