12.5 on the T3 won't be much for you, but Kaz is right, it'll be a good start. Ramp up the dosages every 3 days, with a day break in between. If your thyroid is healthy, it shouldn't take much based on what you are trying to accomplish. Don't run it for more than say 6 weeks. (Ramp up, and Ramp down) There's been quite a few studies done on Thyroid shutdown and the "Gate Slam", but they are pretty much in contradictory to each other. Some say the Thyroid slams shut, others say it rebounds on it's own upon cessation. Best to monitor for yourself and see how you tolerate it.
For now, probably keep the doses down to 50/100mcg at the highest. You don't have to go there if you're feeling good at say 37.5/75... just set a limit for yourself and stick to it.