What dose of l-methylfolate works best for?

If You get an adequate response at 7.5 mg then stick with it. Otherwise 15 mg should do the trick. My understanding is that doses above 15 mg don't prove more effective than 15 mg. L-methylfolate is well tolerated in terms of side effects and long term use. You can dose in the 100s of mg without much concern. But again, if 15 mg does not help with depression then it is likely that doses above that won't either. Make sure You purchase from a reliable source such as Methyl Pro. Although a tad pricey, NOW foods 5000 mcg l-methylfolate is a good option as well. You would need to take 3 per day to get 15 mg.