Hips and waist seem to be about the slowest to change. Losing weight early will help. It's hard to get rid of belly fat after HRT. Set yourself in for the long haul. HRT can take 5 to 7 years to complete.
As to converting men's sizes to women's sizes, you don't really. Women's sizes are inconsistent from manufacturer to manufacturer. They can be cut differently because women's clothes aren't baggy like men's. Also there's no correlation between women's waists and women's butts.
You have to pick things out and try them on. This is a feature, not a bug.

The spy who came in from the cold in the War Between the Sexes.