So, I get that you find the anchor for the I chord based on where the root would be if you were fretting an E-shape barre. Then you slide down/up a half tone for the IV/V chords. I'm a little lost trying to figure how you'd do this for a blues in E, though. The above convention would have me doing a single fretted note plus an open for the I chord, and nowhere to go for the IV. Clearly anchoring this progression involves some theory cleverness I do not yet possess. Little help from my friends?