If you are past the throttle chop instinct (that is, not doing that) I'd suggest a bare minimum of -4 on the bottom. Any less and it may not want to come down without loosing too much headspeed as you'd be at idle to get 0 pitch if that was set to be minimum. I think a good AP setup is -6 at the bottom and +12 at the top. If you need to soften the middle use some expo.
Also you may want to add a couple of brass wheel collars to the flybar near the paddles. The extra weight will soften the cyclic. about 6 deg cyclic should be plenty, once again some expo will help.
Try about 1600-1700 RPM. Lower headspeed will make a big difference. Set the same pitch curve in all settings and adjust your throttle curve for the different flight modes to raise and lower the HS. Best setup will giuve you a constant HS in a given flight mode through out the pitch curve i.e. 1550 norm, 1650 IDLE1, 1750 IDLE2 from hover to full throttle.
I've flown as low at 1400 RPM, gets really mushy there but I think it's kind of fun doing lazy figure 8's 10' off the ground with it burpling along. BUT I also have some throttle mixed into cyclic so 1400 didn't fall off when turning. I wouldn't recommend going this low HS since it leaves very little margin for getting out of trouble if you flame out or jam the sticks in the corners.