I've found that for most of my work I don't use an electronic scale for my spindle travel. I don't use spindle travel to change my depth of cut when milling; I raise the knee. I lock the spindle and leave it set for milling operations. I only use spindle travel for drilling, counter boring, boring, etc..
That said, I do like the idea of having an electronic measurement on my spindle travel for those times when it is handy, but mounting a full blown DRO scale on the mill head wasn't something I desired. I don't want all that bulk (scale, head, covers, cable, etc) mounted on the front of my head.
So, I compromised and use a caliper type scale on my machine. This is a lot smaller and easier to install. It serves the purpose for my infrequent use. TPACtools sells (2) different 4 axis DRO kits... one with the caliper scale for the spindle and one with a full DRO scale. I went with the caliper type.
Here's a picture of my Bridgeport install.
I still can use my rigid quill stop, which I set for counterboring and repetitive depth work. I set the stop and lower the quill. Raise the knee up until the tool touches; move the quill up out of the way and set depth with my knee Z DRO.
This combo works great for my use!
Ted