Which kit is this and what is the wing area? This will give a clue on wingloading. I assume you have the batt as far forward as possible? Thunderbolt's have a fairly large forgiving wing as compared to some other warbirds. If you state your model, others may provide first hand experience for you. It is looking like you will probably want at least a .61 at 7 lbs, although I flew a Senior Kadet with a .46 at almost 11lbs.