Closet Flange Question

I am tiling a small bathroom and the old ABS closet flange was not level. It was out about 3/8" from front to back. Plus it was not screwed to the subfloor very well. None of the screws were biting into the subfloor because the hole was cut to large. I decided to put in a new flange and and replace the subfloor around the flange. What I found out is that the 3" drain pipe to the flange is going slightly downhill thus making the flange not level. I really dont see anyway to correct this. I guess it worked ok for 30 years it should be ok. My other question, is there a rull of thumb for cutting the pipe going to the flange so the flange will set on the floor? I dont want to cut the pipe to short or too long? I currently have the flange pipe coming out of the floor until I finish tiling.
Thanks
Doug

You can glue the pipe into the flange then dry fit and cut until it fits right. Many times you can screw the flange down flat when it is pitched like that, now that you have sub floor to screw to.

The biggest hassle is that you will not get the dry fit pieces to seat all of the way (by design) until you get the glue on there. The glue actually melts the surface layers, and welds the pieces together, allowing it to bottom in the fitting. If you measure the depth of the fitting to the stop, you can determine exactly how much length you need. Dry fit will get close, but you might end up a little short if you are not careful (plus, it is a b**** to get the pieces back apart if you do try to bottom it in the fitting).