Hi keith1961.

@You "...Do you know why the Rocket forum has gone quiet?..."

Good question. Also I'm wondering...

I'm going to echo this question also on NVIL's forum that is managed by Istonia (the creator of R3F along Samardac)

@You : "...I want to know how to constrain extrude to 90 degrees..."

What exactly do you mean ?

If you're talking about "Normal Extrude", this is very simple in R3F.
"Normal Extrude" in fact already extrude a face or a group of faces to 90 degrees.

It's very simple.
1) Pick the face(s) you want to extrude
2) Hit the "a" key
3) With that key pressed push the middle mouse button and start to drag (you can also release the "a" key after you have started to drag)
4) release the middle mouse button when finished

And in this way you can perform a "extrude normal".

Bye!