A standard dose of Aromasin for many at 300mg / week, which is an excellent dose by the way, would be 12.5mg ED. But as Roush points out it may vary greatly for many. Roush takes a TRT dose of 200mg/ week, which would be considered high for many for TRT purposes, but that too greatly varies for many. For me I take 125mg test E / week (half monday evening, and half Friday morning) and take 12.5mg with my injections which is 25mg / week.
If I were you I'd start at 12.5mg ED at 300mg test / week. At that dose you will not crash your E2, and without bloodwork you are just feeling it out. Over time you should be able to dial it in by feeling if you can't get bloodwork to confirm where you are at. You'd need a sensistive e2 assay to know for sure, but the beauty of Aromasin is that it is a mild suicidal AI and very hard to crash your estrogen. So feel it out, bud.