70-90 is what I usually recommend when you are not using dichlor or trichlor for chlorine. If your CYA levels are reasonably low you can often go lower, some people go down to 50 or so. Adding borates to 30-50 ppm allows you to lower TA even further. What level is appropriate for your pool depends on what you are using for chlorine, what the surface of the pool is made of (plaster, vinyl, etc) and where your other levels are.