It was test and tune so I wasn't worried about reaction time or the lights.
Instead of feathering the throttle and trying to hold an rpm, floor it.
When the revs get to around 4500 "slip" the clutch. This takes practice. You have to slip it fast enough to allow it to grab but not so fast that you're just dumping the clutch. If the clutch grabs but the engine bogs, try a little higher rpm. If the engine doesn't bog but the clutch does't grab (first let off right away) try a lower rpm. It won't take long to get it down.
Keep it floored and be ready to shift to second. It comes quick. When you do it right, you'll know it. You're head will probably smack the headrest.
The reason for flooring it: When you floor it, the throttle is fully open (duh) and that means that there is the maximum amount of exhaust gas exiting the engine and spooling the turbo. This means you'll make more boost at launch than if you just feather the throttle, thus creating more power.