An excellent device. Within reason the thicker the perspex the better but I made mine from 1/4" as that was what I had in in the box intending to replace it once I got something thicker. Not ideal but workable so yet another temporary job becomes more permanent than the proper one.
I used a 1 1/4" square mirror and 1 1/4" cube base block, a little smaller than Teds original. Works fine with my Smart & Brown 1024. On a Myford I'd be tempted to go down to a 1" mirror and 1" cube base. I gave mine extra lines so it can be used standing on cross slide, saddle and bed which can be very helpful if tool resetting is needed with a job in place. The mirror is attached via a nice knurled thumbscrew so it can be easily moved as required. Sometimes its just as easy to use it as a simple line gauge bringing the tool tip up so it almost touches the perspex.
You do need plenty of light, preferably from a source with a reasonable range of adjustment so that things are illuminated from best angles. With simple flat overall lighting the contrast between tool, background and lines can be somewhat lacking. Especially for older eyes like mine. A really good, clean mirror makes visibility far better too. Make a cover so it doesn't get dirty. My mirror was adequate rather than excellent and I didn't make a cover trusting that carefully putting away would suffice when backed up by an annual clean with Isopropyl Alcohol. It doesn't really. Sans cover its amazing how grubby it gets. Slow performance drop off means its easy its easy to forget how good the image is when clean and get in the habit of struggling on with a poor view.
Clive.